Selective field ionization of high Rydberg states: Application to zero-kinetic-energy photoelectron spectroscopy

Pulsed-ramped-field-ionisation zero-kinetic-energy photoelectron spectroscopy of the metastable rare-gas atoms Ar, Kr and Xe

The photoionisation of the metastable rare-gas atoms Rg = Ar, Kr and Xe is investigated at the Rg+ […](ns)2(np)5 2P3/2 ← Rg[…](ns)2(np)5((n + 1)s) 13P2 photoionisation threshold (n = 3, 4 and 5 for Ar, Kr and Xe) using the technique of pulsed-ramped-field-ionisation zero-kinetic-energy (PRFI-ZEKE) photoelectron spectroscopy. This technique, which monitors the field ionisation of high Rydberg states induced by a slowly growing electric-field ramp after a prepulse of opposite polarity, was recently introduced to record photoelectron spectra with high resolution and high sensitivity [Harper, Chen, Boyé-Péronne and Gans, Phys. Chem. Chem. Phys., 2022, 117, 9353]. A tunable UV laser system with a bandwidth of 30 MHz is used here to establish the factors determining the resolution and overall accuracy of this new method. In particular, the effects of stray electric fields, and of the magnitude of the prepulse and the field ramp on PRFI-ZEKE photoelectron spectra are studied by combining experiments with numerical simulations of the field-ionisation of high Rydberg states and of the electron flight times from the ionisation spot to the detector. A spectral resolution of 0.05 cm-1 is demonstrated in the photoelectron spectrum of metastable Ar.

Elucidating the photoprotective properties of natural UV screening agents: ZEKE-PFI spectroscopy of methyl sinapate

As a prominent derivative of a natural sunscreen, methyl sinapate is an ideal candidate to provide fundamental insight into strategies on how to come to a rational design of artificial sunscreen filters with improved photoprotective properties. Here, static and time-resolved Zero Kinetic Energy-Pulsed Field Ionization (ZEKE-PFI) photoelectron spectroscopy has been used to study the spectroscopy and decay pathways of its electronically excited states. We find that different conformers are subject to distinct structural changes upon electronic excitation, and trace the structural changes that occur upon excitation back to the character of the LUMO. Ionization efficiency spectra in combination with pump-probe ZEKE-PFI spectra are consistent with the conclusion that the long-lived electronically excited state observed in the decay of the lowest excited singlet state is the lowest excited triplet state. Concurrently with providing information on the electronically excited states, the studies allow for a detailed characterization of the spectroscopic properties of the ground state of the radical ion, which is important in the context of the use of cinnamates in nature as antioxidants. Our studies determine the adiabatic ionization energies of the syn/cis, anti/cis and anti/trans conformers as 60 291.1 ± 0.5, 60 366.9 ± 0.5 and 60 503.9 ± 1.0 cm^-1, respectively, and provide accurate vibrational fequencies of low-frequency modes of the molecular ion in its electronic ground state. Finally, the studies emphasize the important role of vibrational and electronic autoionization processes that start to dominate the ionization dynamics in non-rigid molecules of the present size.

Pulsed-ramped-field-ionization zero-kinetic-energy photoelectron spectroscopy: a methodological advance

A new experimental method has been developed to record photoelectron spectra based on the well-established pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy technique and inspired by the data treatment employed in slow photoelectron spectroscopy. This method has been successfully applied to two well-known systems: the X⁺²Π_g,1/2(v⁺ = 0) ← X¹Σ+g(v = 0) and the X⁺¹Σ⁺(v⁺ = 2) ← X²Π_1/2(v = 0) ionizing transitions of CO₂ and NO, respectively. The first results highlight several advantages of our technique such as an improved signal-to-noise ratio without degrading the spectral resolution and a direct field-free energy determination. The data obtained for NO indicate that this method might be useful for studying field-induced autoionization processes.

Innovative mass spectrometer for high-resolution ion spectroscopy

Conventional ion spectroscopy is inapplicable for ions produced in low concentrations or with low spectral resolutions. Hence, we constructed a high-resolution vacuum ultraviolet mass-analyzed threshold ionization (HR VUV-MATI) spectrometer composed of a four-wave frequency mixing cell capable of generating long-lasting and intense VUV laser pulses of ∼1 × 10¹⁰ photons/pulse at wavelengths of 123.6-160.0 nm, a space-focused linear time-of-flight photoionization chamber with a new ion-source assembly, and a compact molecular beam chamber with a temperature-controlled pulsed nozzle for ion spectroscopy. The ion-source assembly and pulsing schemes enabled an ∼15-μs-delayed but extremely weak pulsed-field-ionization of the molecules in the zero-kinetic-energy (ZEKE) states and first-order space focusing of the generated MATI ions. These ZEKE states were effectively generated by a minute electric jitter from the high-lying Rydberg states, which were initially prepared via VUV photoexcitation. The spectral and mass resolutions (∼5 cm^-1 and 2400, respectively) and the signal strength were simultaneously enhanced using this spectrometer. Moreover, it could be used to measure the fine vibrational spectrum from the zero-point level of the cation and the exact adiabatic ionization energy of the neutral molecule. Additionally, it could be used to measure the appearance energies of the photoproducts and elucidate the vibrational structures of the cationic isotopomers, utilizing other pulsing schemes. Furthermore, this spectrometer could be used to analyze the congested vibrational spectrum of a cation with multiple conformations. Thus, the HR VUV-MATI spectrometer-a potential alternative to photoelectron spectrometers-can be used to analyze the conformational structure-dependent reactivities.

Spectroscopic characterization of a thermodynamically stable doubly charged diatomic molecule: MgAr2

Although numerous doubly positively charged diatomic molecules (diatomic dications) are known from investigations using mass spectrometry and ab initio quantum chemistry, only three of them, NO²⁺, N₂²⁺ and DCl²⁺, have been studied using rotationally resolved optical spectroscopy and only about a dozen by vibrationally resolved double-ionization methods. So far, no thermodynamically stable diatomic dication has been characterized spectroscopically, primarily because of experimental difficulties associated with their synthesis in sufficient densities in the gas phase. Indeed, such molecules typically involve, as constituents, rare-gas, halogen, chalcogen, and metal atoms. We report here on a new approach to characterize molecular dications based on high-resolution photoelectron spectroscopy of the singly charged parent molecular cation and present the first spectroscopic characterization of a thermodynamically stable diatomic dication, MgAr²⁺. From the fully resolved vibrational and partially resolved rotational structures of the photoelectron spectra of ²⁴MgAr⁺ and ²⁶MgAr⁺, we determined the potential-energy function of the electronic ground state of MgAr²⁺, its dissociation (binding) energy (D₀ = 10 690(3) cm⁻¹), and its harmonic (ω_e(²⁴MgAr²⁺) = 327.02(11) cm⁻¹) and anharmonic (ω_ex_e(²⁴MgAr²⁺) = 2.477(15) cm⁻¹) vibrational constants. The analysis enables us to explain quantitatively how the strong bond arises in this dication despite the fact that Ar and Mg²⁺ both have a full-shell rare-gas electronic configuration.

We present a new method to study doubly charged molecules relying on high-resolution spectroscopy of the singly charged parent cation, and report on the first spectroscopic characterization of a thermodynamically stable diatomic dication, MgAr²⁺.

PFI-ZEKE-photoelectron spectroscopy of N2O using narrow-band VUV laser radiation generated by four-wave mixing in Ar using a KBBF crystal

A new nonlinear optical scheme relying on sum-frequency mixing in a KBe₂BO₃F₂ crystal has been used to generate intense, broadly tunable, narrow-bandwidth, coherent vacuum-ultraviolet (VUV) radiation beyond 16 eV by resonance-enhanced four-wave mixing in Ar. The VUV radiation was used to record high-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the N₂O⁺ A⁺ ← N₂O X photoionizing transition in the wave-number range from 132 000 cm^-1 to 135 000 cm^-1. The rotational structure of almost all vibrational levels of the A⁺ state with vibrational term values up to 2700 cm^-1 could be resolved, and improved values of the first two adiabatic ionization energies of N₂O, corresponding to the formation of the X^{+ 2}Π_3/2(000) J⁺ = 3/2 and A^{+ 2}Σ⁺(000) N⁺ = 0 levels of N₂O⁺ from the X ¹Σ⁺(000) J″ = 0 ground state [103 969.30(12) cm^-1 and 132 197.70(12) cm^-1, respectively], were derived. The rotational intensity distributions of the bands were found to depend strongly on the value of the vibrational angular momentum of the ionic levels. The vibrational structure is discussed in terms of previously reported effective-Hamiltonian analyses.

Communication: Heavy-Rydberg states of HD and the electron affinity of the deuterium atom

The electron affinity of the deuterium atom has been determined to be 6086.81(27) cm^-1 from a measurement of the difference between the D⁺ + H^- and H⁺ + D^- ion-pair dissociation energies and a thermochemical cycle involving the electron affinity of H and the ionization energies of H and D. Heavy-Rydberg states and the ion-pair dissociation thresholds of HD were accessed with good efficiency using a three-photon excitation sequence through the B Σu+1 (v = 22, N = 1) and H¯ Σg+1 (v = 9, N = 0) intermediate levels and the threshold positions were determined using the method of threshold-ion-pair-production spectroscopy.

Spin-orbit interaction and Renner-Teller effect in HCCCCH+ studied by high-resolution photoelectron spectroscopy

The photoelectron spectra of the X^{+ 2}Π_g ← X ¹Σ photoionizing transition of diacetylene (HCCCCH) and d2-diacetylene (DCCCCD) have been recorded at high resolution using the technique of pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The partially resolved rotational structure of the origin band of the spectra of HCCCCH and DCCCCD has enabled the determination of the adiabatic ionization energies of HCCCCH [E_ad = 82072.2(5) cm^-1] and DCCCCD [E_ad = 82090.0(10) cm^-1] and of the spin-orbit coupling constant [A = -31.1(4) cm^-1] of the ground vibronic state of HCCCCH⁺, which is smaller than the value of -33.3 cm^-1 commonly used since the work of Callomon (J. H. Callomon, Can. J. Phys., 1956, 34, 1046). Several excited vibrational levels of HCCCCH⁺ and DCCCCD⁺, including some affected by the Renner-Teller effect and Fermi interactions, have been observed and the fundamental wavenumber of the mode ν₉ has been determined in both HCCCCH⁺ (200.0(10) cm^-1) and DCCCCD⁺ (192.6(20) cm^-1). Possible assignments for several of these levels are discussed and deficiencies in the current understanding of the energy-level structure of the radical cation of diacetylene are pointed at.

Structure and dynamics of the radical cation of ethane arising from the Jahn-Teller and pseudo-Jahn-Teller effects

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of C₂H₆ has been recorded in the region of the adiabatic ionization threshold. The partially rotationally resolved spectrum indicates the existence of several vibronic states of C₂H₆⁺ with less than 600 cm^-1 of internal excitation. The analysis of the rotational structures assisted by ab initio calculations enabled the determination of the adiabatic ionization energy of C₂H₆ and the investigation of the structure and dynamics of C₂H₆⁺ at low energies. The ground state of C₂H₆⁺ is found to be a ²A_g state of diborane-like structure with strongly mixed (a_1g)^-1 and (e_g)^-1 configurations. The vibrational structure reveals the importance of large-amplitude nuclear motions involving the diborane distortion modes, the C-C stretching motion, and the internal rotation at elongated C-C distances. The spectrum is analyzed in the light of the information obtained in earlier studies of C₂H₆⁺ by ab initio quantum chemistry, EPR spectroscopy and photoelectron spectroscopy.

Intrabeam Scattering for Ultracold Collisions

We demonstrate a method to probe cold and ultracold chemistry in a single molecular beam. The approach exploits beam slippage, the velocity difference of different species in the same beam, to establish the relative velocity. Average collision energies of 2.5 mK are achieved but with a spread of 100% or more. However, by implementing a dual-slit chopper that can separately fix the velocities of the two species at the interaction region, we achieve precise control over the relative velocity and narrow its spread. Relative velocities of 7-10 ± 1.1 m/s are achieved with an angular divergence less than 0.25°. In the present study, we observe l-changing collisions occurring between Xe Rydberg atoms and Xe ground state atoms at subKelvin temperatures. We show that in this case the collision energies are tunable between 200 to 450 mK with a root-mean-square deviation of ∼18%. Application of the method to other species and access to much lower energies is straightforward.

Vacuum-ultraviolet frequency-modulation spectroscopy

Frequency-modulation (FM) spectroscopy has been extended to the vacuum-ultraviolet (VUV) range of the electromagnetic spectrum. Coherent VUV laser radiation is produced by resonance-enhanced sum-frequency mixing (ν_VUV=2ν_UV+ν₂) in Kr and Xe using two near-Fourier-transform-limited laser pulses of frequencies ν_UV and ν₂. Sidebands generated in the output of the second laser (ν₂) using an electro-optical modulator operating at the frequency ν_mod are directly transferred to the VUV and used to record FM spectra. Demodulation is demonstrated both at ν_mod and 2ν_mod. The main advantages of the method compared to VUV absorption spectroscopy are its background-free nature, the fact is that its implementation using table-top laser equipment is straightforward and that it can be used to record VUV absorption spectra of cold samples in skimmed supersonic beams simultaneously with laser-induced-fluorescence and photoionization spectra. To illustrate these advantages, we present VUV FM spectra of Ar, Kr, and N₂ in selected regions between 105000 cm^-1 and 122000 cm^-1.

Observation and Calculation of the Quasibound Rovibrational Levels of the Electronic Ground State of H2+

Although the existence of quasibound rotational levels of the X^{+} ^{2}Σ_{g}^{+}  ground state of H_{2}^{+} was predicted a long time ago, these states have never been observed. Calculated positions and widths of quasibound rotational levels located close to the top of the centrifugal barriers have not been reported either. Given the role that such states play in the recombination of H(1s) and H^{+} to form H_{2}^{+}, this lack of data may be regarded as one of the largest unknown aspects of this otherwise accurately known fundamental molecular cation. We present measurements of the positions and widths of the lowest-lying quasibound rotational levels of H_{2}^{+} and compare the experimental results with the positions and widths we calculate using a potential model for the X^{+} state of H_{2}^{+} which includes adiabatic, nonadiabatic, relativistic, and radiative corrections to the Born-Oppenheimer approximation.

Absorption, autoionization, and predissociation in molecular hydrogen: High-resolution spectroscopy and multichannel quantum defect theory

Absorption and photoionization spectra of H2 have been recorded at a resolution of 0.09 and 0.04 cm(-1), respectively, between 125,600 cm(-1) and 126,000 cm(-1). The observed Rydberg states belong to series (n = 10 - 14) converging on the first vibrationally excited level of the X (2)Σ(g)(+) state of H2(+), and of lower members of series converging on higher vibrational levels. The observed resonances are characterized by the competition between autoionization, predissociation, and fluorescence. The unprecedented resolution of the present experimental data leads to a full characterization of the predissociation/autoionization profiles of many resonances that had not been resolved previously. Multichannel quantum defect theory is used to predict the line positions, widths, shapes, and intensities of the observed spectra and is found to yield quantitative agreement using previously determined quantum defect functions as the unique set of input parameters.

The cyclopropene radical cation: rovibrational level structure at low energies from high-resolution photoelectron spectra

The cyclopropene radical cation (c-C3H₄⁺) is an important but poorly characterized three-membered-ring hydrocarbon. We report on a measurement of the high-resolution photoelectron and photoionization spectra of cyclopropene and several deuterated isotopomers, from which we have determined the rovibrational energy level structure of the X⁺ (2)B2 ground electronic state of c-C3H₄⁺ at low energies for the first time. The synthesis of the partially deuterated isotopomers always resulted in mixtures of several isotopomers, differing in their number of D atoms and in the location of these atoms, so that the photoelectron spectra of deuterated samples are superpositions of the spectra of several isotopomers. The rotationally resolved spectra indicate a C(2v)-symmetric R0 structure for the ground electronic state of c-C3H₄⁺. Two vibrational modes of c-C3H₄⁺ are found to have vibrational wave numbers below 300 cm(-1), which is surprising for such a small cyclic hydrocarbon. The analysis of the isotopic shifts of the vibrational levels enabled the assignment of the lowest-frequency mode (fundamental wave number of ≈110 cm(-1) in c-C3H₄⁺) to the CH2 torsional mode (ν₈⁺, A2 symmetry) and of the second-lowest-frequency mode (≈210 cm(-1) in c-C3H₄⁺) to a mode combining a CH out-of-plane with a CH2 rocking motion (ν₁₅⁺, B2 symmetry). The potential energy along the CH2 torsional coordinate is flat near the equilibrium structure and leads to a pronounced anharmonicity.

The X+ 2Πg, A+ 2Πu, B+ 2Δu, and a+ 4Σu(-) electronic states of Cl2(+) studied by high-resolution photoelectron spectroscopy

Partially rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the three isotopomers ((35)Cl2, (35)Cl(37)Cl, and (37)Cl2) of Cl2 have been recorded in the wavenumber ranges 92,500-96,500 cm(-1), corresponding to transitions to the low vibrational levels of the X(+) (2)Πg (Ω = 3∕2, 1∕2) ground state of Cl2 (+), and 106,750-115,500 cm(-1), where the a(+) (4)Σu (-)←X(1)Σg (+), A(+) (2)Πu←X(1)Σg (+), and B(+) (2)Δu←X(1)Σg (+) band systems overlap with transitions to high vibrational levels (v(+) > 25) of the X(+) state. The observation of Franck-Condon-forbidden transitions to vibrational levels of the X(+) state of the cation with v(+) ≥ 25 is rationalized by a mechanism involving vertical excitation of predissociative Rydberg states of mixed singlet-triplet character with an A(+) ion core which are coupled to Rydberg states converging to high-v(+) levels of the X(+) state. The same mechanism is proposed to also be responsible for the observation of Cl(+) - Cl(-) ion pairs and quartet states in the photoionization of Cl2. The potential energy function of the X(+) state of Cl2 (+) was determined in a direct fit to the experimental data. Transitions to vibrational levels of the A(+) (2)Πu, 3∕2 and B(+) (2)Δu, 3∕2 states of Cl2 (+) could be identified using the results of a recent analysis of the strong perturbation between the A(+) (2)Πu, 3∕2 and B(+) (2)Δu, 3∕2 states of Cl2 (+) observed in the A(+) - X(+) band system [Gharaibeh et al., J. Chem. Phys. 137, 194317 (2012)], and transitions to several vibrational levels of the upper spin-orbit component ((2)Πu, 1∕2) of the A(+) state were detected in the photoelectron spectrum of Cl2 (+). The a(+) (4)Σu (-)←X(1)Σg (+) photoelectron band system, which is nominally forbidden by single-photon ionization from the ground state was also observed for the first time and its vibrational and spin-orbit structures were analyzed. The (4)Σu (-) state is split into two spin-orbit components with Ω = 1∕2 and Ω = 3∕2, separated by 37.5 cm(-1). The vibrational energy level structure of both components is regular, which indicates that the splitting results from the interaction with one or more distant ungerade Ω = 1∕2 or Ω = 3∕2 electronic states.

Spin-orbit and vibronic coupling in the ionic ground state of iodoacetylene from a rotationally resolved photoelectron spectrum

The X(+ 2)Π ← X (1)Σ(+) photoionizing transition of iodoacetylene (HC2I) has been investigated by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The resolution of the rotational structure of the spectra and its analysis provided information on the structure of the HC2I(+) cation and the photoionization dynamics of HC2I. In the ground electronic (2)Π state, the HC2I(+) cation is found to be linear and subject to a strong spin-orbit coupling. The first adiabatic ionization energy of HC2I and the spin-orbit splitting of the X(+ 2)Π ground state of HC2I(+) were determined to be EI(HC2I)/hc = 78296.5(2) cm(-1) and Δν̃SO = 3257(1) cm(-1), respectively. The large spin-orbit interaction almost entirely masks the Renner-Teller effect, which is only detectable through the observation of the nominally forbidden transition to the first excited level (5(1)) of the HCC-I bending mode ν5. The interaction of ∼2 cm(-1) observed between the 5(1) levels of (2)Σ1/2 and (2)Δ5/2 symmetry is attributed to a vibronic interaction with the B (2)Σ(+) electronic state of HC2I(+). The spin-orbit energy level structure of tri- and tetra-atomic molecules subject to the Renner-Teller effect and spin-orbit coupling is discussed for the two limiting cases where the spin-orbit-coupling constant is much smaller and much larger than the bending-mode frequencies.

Rotationally resolved PFI-ZEKE photoelectron spectroscopic study of the low-lying electronic states of ArXe+

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X 1/2, A(1) 3/2, and A(2) 1/2 electronic states of the ArXe(+) molecular ion have been recorded following resonant (1+1') two-photon excitation via selected rovibrational levels of the C 1 and D 0(+) states of selected isotopomers of the ArXe molecule. Using rovibronic selection and propensity rules for the photoionization out of these intermediate molecular states enabled the determination of the parity of the molecular-ion levels and of the magnitude and sign of the Ω-doubling constants of the coupled X 1/2 (p ≈ 4B) and A(2) 1/2 (p ≈ -2B) states of ArXe(+). The results indicate that these molecular-ion states can be approximately described using Mulliken's second variant of Hund's angular momentum coupling case (c), for which J(a), the total electronic and spin angular momentum of the two atoms, is a good quantum number (semi-united atom). The analysis of the rotational structure enabled the derivation of improved values of the dissociation energies, equilibrium distances, and molecular constants for the X 1/2, A(1) 3/2, and A(2) 1/2 states of ArXe(+).

Torsional vibrational structure of the propene radical cation studied by high-resolution photoelectron spectroscopy

The pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X̃(+2)A(")←X̃(1)A' transition of CH(3)CHCH(2) (propene), CD(3)CDCD(2), and several partially deuterated isotopomers have been recorded in the region of their adiabatic ionization thresholds and up to 2000 cm(-1) of internal energy of the cations. The vibrational structure has been assigned on the basis of the frequency shifts resulting from deuteration of selected sites of the propene molecule. Two highly anharmonic progressions have been identified and assigned to the two torsional modes of the propene cation, the methyl and methylene torsions. The positions of the torsional levels could be approximately reproduced using one-dimensional models, allowing a semi-quantitative description of the potential energy surface along each torsional coordinate. The observation of forbidden vibrational bands and the analysis of their partially resolved rotational contours reveal the importance of the vibronic coupling between the X̃(+2)A(") and the Ã(+2)A(') states mediated by the methylene (ν(20)) and methyl (ν(21)) torsional modes.

Jahn-Teller effects in molecular cations studied by photoelectron spectroscopy and group theory

The traditional "ball-and-stick" concept of molecular structure fails when the motion of the electrons is coupled to that of the nuclei. Such a situation arises in the Jahn-Teller (JT) effect which is very common in open-shell molecular systems, such as radicals or ions. The JT effect is well known to chemists as a mechanism that causes the distortion of an otherwise symmetric system. Its implications on the dynamics of molecules still represent unsolved problems in many cases. Herein we review recent progress in understanding the dynamic structure of molecular cations that have a high permutational symmetry by using rotationally resolved photoelectron spectroscopy and group theory. Specifically, we show how the pseudo-Jahn-Teller effect in the cyclopentadienyl cation causes electronic localization and nuclear delocalization. The fundamental physical mechanisms underlying the vaguely defined concept of "antiaromaticity" are thereby elucidated. Our investigation of the methane cation represents the first experimental characterization of the JT effect in a threefold degenerate electronic state. A special kind of isomerism resulting from the JT effect has been discovered and is predicted to exist in all JT systems in which the minima on the potential-energy surface are separated by substantial barriers.

Pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy of metastable He2: Ionization potential and rovibrational structure of He2+

A supersonic beam of metastable He(*) atoms and He(2) (*) a (3)Sigma(u) (+) molecules has been generated using a pulsed discharge at the exit of a pulsed valve prior to the gas expansion into vacuum. Pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the He(2) (+) X(+) (2)Sigma(u) (+) (v(+)=0-2)<--He(2) (*) a (3)Sigma(u) (+) (v(")=0-2) transitions and photoionization spectra of He(2) (*) in the vicinity of the lowest ionization thresholds have been recorded. The energy level structures of (4)He(2) (+) X(+) (2)Sigma(u) (+) (v(+)< or =2,N(+)< or =23) and (3)He(2) (+) X(+) (2)Sigma(u) (+) (v(+)=0,N(+)< or =11) have been determined, and an accurate set of molecular constants for all isotopomers of He(2) (+) has been derived in a global analysis of all spectroscopic data reported to date on the low vibrational levels of He(2) (+). The analysis of the photoionization spectrum by multichannel quantum defect theory has provided a set of parameters describing the threshold photoionization dynamics.

High resolution spectroscopy for the A (2)A(1) state of CH(3)I(+) by mass-analyzed threshold ionization/photodissociation

Photodissociation of CH(3)I(+) in the ground vibronic state generated by mass-analyzed threshold ionization resulted in a superb spectrum for the first excited electronic state (A (2)A(1)) with hardly any spurious peak. Rotational structure in the spectrum could be resolved by using a single mode laser. This structure for one vibronic band, 2(1)3(1)6(1), was analyzed with the assumption of Hund's case (a) scheme both in the ground and excited electronic states.

Jahn-Teller effect in tetrahedral symmetry: Large-amplitude tunneling motion and rovibronic structure of CH4+ and CD4+

The energy level structures of the ground vibronic states of 12CH4+, 13CH4+, and 12CD4+ have been measured by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy. The nuclear spin symmetries of the tunneling-rotational levels have been determined in double-resonance experiments via selected rotational levels of the v3=1 and v3=2 vibrational levels of the X 1A1 ground state of CH4. The energy level structures of 12CH4+, 13CH4+, and 12CD4+ have been analyzed with an effective tunneling-rotational Hamiltonian. The analysis together with a group theoretical treatment of the Tx(e+t2) Jahn-Teller effect in the Td(M) group prove that the equilibrium geometry of 12CH4+, 13CH4+, and 12CD4+ has C2v symmetry and characterize the pseudorotational dynamics in these fluxional cations. The tunneling behavior is discussed in terms of the relevant properties of the potential energy surface, some of which have been recalculated at the CCSD(T)/cc-pVTZ level of ab initio theory.

Rovibrational photoionization dynamics of methyl and its isotopomers studied by high-resolution photoionization and photoelectron spectroscopy

High-resolution photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of CH(3), CH(2)D, CHD(2), and CD(3) have been recorded in the vicinity of the first adiabatic ionization threshold following single-photon excitation from the ground neutral state using a narrow-bandwidth vacuum-ultraviolet laser. The radicals were produced from the precursor molecules methyl-bromide, methyl-iodide, dimethyl-thioether, acetone, and nitromethane by 193 nm excimer photolysis in a quartz capillary and were subsequently cooled to a rotational temperature T(rot) approximately equal to 30 K in a supersonic expansion. Nitromethane was identified as a particularly suitable photolytic precursor of methyl for studies by photoionization and threshold photoelectron spectroscopy. Thanks to the cold rotational temperature reached in the supersonic expansion, the rotational structure of the threshold ionization spectra could be resolved, and the photoionization dynamics investigated. Rydberg series converging on excited rotational levels of CH(3) (+) could be observed in the range of principal quantum number n=30-50, and both rotational autoionization and predissociation were identified as decay processes in the threshold region. The observed photoionization transitions can be understood in the realm of an orbital model for direct ionization but the intensity distributions can only be fully accounted for if the rotational channel interactions mediated by the quadrupole of the cation are considered. Improved values of the adiabatic ionization thresholds were derived for all isotopomers [CH(3): 79 356.2(15) cm(-1), CH(2)D: 79 338.8(15) cm(-1), CHD(2): 79 319.1(15) cm(-1), and CD(3): 79 296.4(15) cm(-1)].

Bending energy level structure and quasilinearity of the X̃+B13 ground electronic state of NH2+

The bending level structure of the quasilinear X+ 3B1 ground electronic state of the amidogen cation NH2+ was studied by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy using a near-infrared vacuum-ultraviolet two-photon ionization sequence via selected rovibronic levels of the A 2A1 state of NH2. The careful selection of the intermediate levels permitted to optimize the transition intensities to the lowest vibrational levels of the cation in the photoionization step and to overcome the low sensitivity of previously employed single-photon ionization schemes. For the first time, all bending levels of the cationic ground state with quantum numbers upsilon2,lin + < or =4, N+ < or =4, and /K+/ < or =2 could be observed, enabling a detailed characterization of the large-amplitude bending vibration. The rotational structure corresponds to that of an effectively linear molecule in all observed vibrational levels. The bending vibrational structure which shows marked deviations from a harmonic behavior was analyzed in terms of a semirigid bender model. The bending potential function was obtained from a fit to the experimental data. The height of the barrier at the linear geometry and the bond angle at the potential minimum were determined to be 231.8(22) cm(-1) and 152.54(4) degrees , respectively, and all bending levels are located above the maximum of the barrier.

One-Photon Mass-Analyzed Threshold Ionization Spectroscopy (MATI) of trans-Dichloroethylene (trans-C2H2Cl2): Cation Structure Determination via Franck−Condon Fit

One-photon mass-analyzed threshold ionization (MATI) spectrum of trans-C(2)H(2)Cl(2) was obtained by using vacuum ultraviolet radiation generated by four-wave mixing in Kr. The ionization energy determined from the position of the 0-0 band in the spectrum was 9.6306 +/- 0.0006 eV. Ten vibrational fundamentals for the cation were identified. The spectrum also displayed abundant overtones and combinations, most of which could be assigned adequately by comparing with the quantum chemical results. It was found that channel interaction was not important for this system. The equilibrium geometry of the cation was estimated through the Franck-Condon fit.