Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of vinyl bromide: Franck–Condon analysis and vibrational assignment

Multireference configuration interaction study of the vibronic transitions and photodissociation of vinyl bromide and vinyl chloride radical cations in the second excited state

Recent development of techniques to obtain vibronically resolved spectra of radical cations of organic molecules provides detailed information regarding molecular structure and dynamics in the excited states that lead to photodissociation and/or ultrafast nonradiative decay through conical intersections. In this paper, the vibronic transitions and photodissociation of radical cations of vinyl bromide (CH(2)CHBr(+)) and vinyl chloride (CH(2)CHCl(+)) in the second excited state (B) are studied by multireference configuration interaction calculations. Vibronically resolved photofragment yield spectra for the ground state X-B transitions are analyzed using calculated Franck-Condon factors with optimized geometries and normal mode vectors for two states. The relevance of bond dissociation in the B state to the experimental spectra and the dynamics of the C(2)H(3)(+) fragment is explored using calculated potential energy curves for the lower electronic states.

Rotational state selection of a CH3I+ ion beam using vacuum ultraviolet-mass-analyzed threshold ionization spectroscopy: characterization using photodissociation spectroscopy

The A(2)A(1)<--X(2)E(3/2) transition of CH(3)I(+) was investigated by photodissociation (PD) of the cation generated by one-photon mass-analyzed threshold ionization (MATI). Compared to the PD spectrum obtained by excitation of the cation in the main 0-0 band in the MATI spectrum, those obtained by excitation of the cations in the satellite structures showed substantially simplified rotational structures for nondegenerate vibronic bands. Spectral simplification occurred because each satellite consisted mostly of cations with one K quantum number. Spectroscopic constants in the ground vibronic state and in the 2(1)3(5), 2(1)3(8), 3(9), and 3(13) nondegenerate vibrational states in A(2)A(1) were determined via spectral fitting. Also, those in the 2(1)3(n)6(1) (n=1?) degenerate state, which had been reported previously, was improved. The K quantum number in each satellite determined by the present high resolution study was compatible with the prediction by the symmetry selection rule for photoionization. That is, the K quantum number of the ion core in high Rydberg states accessed by one-photon excitation was found to be conserved upon pulsed field ionization. This work demonstrates generation of mass-selected, vibronically selected, and K-selected ion beam by one-photon MATI.

Vibrational spectroscopy and state-specific dissociation dynamics for vinyl chloride cation in the B state

C(2)H(3)(35)Cl+ in the ground vibronic state was generated by one-photon mass-analyzed threshold ionization spectrometry, and its photodissociation in the 461-406 nm range was investigated. Ionization energy to the ground state of C(2)H(3)(35)Cl+ was 10.0062 +/- 0.0006 eV while its B state onset was higher by 2.7456 +/- 0.0003 eV. A vibrational spectrum of the cation in the B state obtained by recording the product ion yield as a function of wavelength was analyzed by referring to the quantum chemical results at the TDDFT/B3LYP/6-311++(df,pd) level. Analysis of product time-of-flight profiles recorded with different laser polarization angles showed that the dissociation pathway for the cation in the B state changed with the vibrational energy, from internal conversion to X and statistical dissociation therein to curve crossing to C and repulsive dissociation therein. B --> C curve crossing seemed to occur along a direction close to the C-Cl bond stretch.

Photodissociation yield spectroscopy of vinyl bromide cation generated by mass-analyzed threshold ionization: Vibrational spectroscopy and decay dynamics in the B̃ state

A new technique [mass-analyzed threshold ionization (MATI)-photodissociation yield spectroscopy] to probe bound excited states of a cation was developed, which measures photodissociation yield of the cation generated by mass-analyzed threshold ionization. A vibrational spectrum of vinyl bromide cation in the (~)B state was obtained using this technique. Optical resolution in the low vibrational energy range of the spectrum was far better than in conventional MATI spectra. The origin of the (~)B state was found at 2.2578+/-0.0003 eV above the first ionization onset. Almost complete vibrational assignment was possible for peaks appearing in the spectrum. Analysis of time-of-flight profiles of C(2)H(3) (+) product ion obtained with different laser polarization angles suggested that photoexcited vinyl bromide cation remained in the (~)B state for several hundred picoseconds prior to internal conversion to the ground state and dissociation therein.

The vibrational structures of furan, pyrrole, and thiophene cations studied by zero kinetic energy photoelectron spectroscopy

The vibrational structures of the electronic ground states ((approximately)X (2)A(2)) of furan, pyrrole, and thiophene cations have been studied by zero kinetic energy (ZEKE) photoelectron spectroscopic method. In addition to the strong excitations of the symmetric a(1) vibrational modes, other three symmetric vibrational modes (a(2), b(1), and b(2)) have been observed unambiguously. These results which cannot be explained by the Franck-Condon principle illustrate that the vibronic coupling and the Coriolis coupling may play important roles in understanding the vibrational structures of the five-membered heterocycle cations. The vibrationally resolved ZEKE spectra are assigned with the assistance of the density function theory calculations, and the fundamental frequencies for many vibrational modes have been determined for the first time. The first adiabatic ionization energies for furan, pyrrole, and thiophene were determined as 8.8863, 8.2099, and 8.8742 eV, respectively, with uncertainties of 0.0002 eV.

Mass-Analyzed Threshold Ionization Study of Vinyl Chloride Cation in the First Excited Electronic State Using Vacuum Ultraviolet Radiation in the 107−102.8 nm Range

Vibrational spectrum of vinyl chloride cation in the first excited electronic state, A2 A', was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. Use of an improved vacuum ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for the MATI signals. From the MATI spectrum, the ionization energy to the A2 A' state of the cation was determined to be 11.6667 +/- 0.0006 eV. Nearly complete vibrational assignment for the MATI peaks was possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the DFT and TDDFT/B3LYP levels with the 6-311++G(3df,3pd) basis set. Geometry of the cation in the A2 A' state was determined by Franck-Condon fitting of the MATI spectrum.

Franck−Condon Simulations of Clusters: Phenol−Nitrogen

Multidimensional Franck-Condon simulations of the resonance enhanced multiphoton ionization (REMPI) and mass-analyzed threshold ionization (MATI) spectra of phenol-nitrogen are obtained from CASSCF, MRCI, and SACCI optimized geometries. In the REMPI simulations, the results are unsatisfactory, as the transitions associated with intermolecular modes are widely underestimated and much less intense than those associated with intramolecular modes. Conversely, the simulations of the MATI spectra show a good similarity to experiment. The best simulations are obtained in both instances from the SACCI optimized geometries. Furthermore, the simulations suggest that the two most prominent Franck-Condon envelopes present in the MATI spectra are due to the sigma and sigma + ngamma' combination bands in accord with the assignments of the MATI spectra of the analogous phenol-carbon monoxide cluster.

A Photoelectron Photoion Coincidence Study of the Vinyl Bromide and Tribromoethane Ion Dissociation Dynamics: Heats of Formation of C2H3+, C2H3Br, C2H3Br+, C2H3Br2+, and C2H3Br3

The threshold photoelectron photoion coincidence (TPEPICO) technique has been used to measure accurate dissociative photoionization onsets of vinyl bromide and 1,1,2-tribromoethane. The reactions investigated and their 0 K onsets are C2H3Br + hnu --> C2H3+ + Br (11.902 +/- 0.008 eV); C2H3Br3 + hnu --> C2H3Br2+ + Br (10.608 +/- 0.008 eV); and (C2H3Br3 + hnu --> C2H3Br+ + 2Br (12.301 +/- 0.035 eV). The vinyl ion heat of formation (Delta(f)H degrees 298K = 1116.1 +/- 3.0 kJ/mol) has been calculated using W1 theory and used as an anchor along with the measured dissociation energies to determine the heats of formation, Delta(f)H degrees 298K, in kJ/mol, of the following bromine-containing species: C2H3Br (74.1 +/- 3.1), C2H3Br+ (1021.9 +/- 3.1), C2H3Br2+ (967.1 +/- 4.0), and C2H3Br3 (53.5 +/- 4.3). These results represent accurate and consistent experimental determinations of heats of formation for these bromine-containing species, which serve to correct the discrepancies in the literature for C2H3Br and C2H3Br+ and provide the first experimental determination for the enthalpies of formation of C2H3Br2+ and C2H3Br3.

Mass-analyzed threshold ionization study of vinyl bromide cation in the first excited electronic state using vacuum-ultraviolet radiation generated by four-wave mixing in Hg

The vibrational spectrum of the vinyl bromide cation in the first excited electronic state A 2A' was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. The use of an improved vacuum-ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for MATI signals. From the MATI spectrum, the ionization energy to the A 2A' state of the cation was determined to be 10.9150+/-0.0006 eV. Nearly complete vibrational assignments for the MATI peaks were possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the density-functional theory (DFT) and time-dependent DFT/B3LYP levels with the 6-311+G(df,p) basis set.

One-photon mass-analyzed threshold ionization spectroscopy of 2-chloropropene (2-C3H5Cl) and its vibrational assignment based on the density-functional theory calculations

A high-quality mass-analyzed threshold ionization (MATI) spectrum of 2-chloropropene, 2-C3H5Cl, is reported. Its ionization energy determined for the first time from the 0-0 band position was 9.5395+/-0.0006 eV. Almost all the peaks in the MATI spectrum could be vibrationally assigned utilizing the frequencies calculated at the B3LYP6-311++G(3df,3pd) level and the Franck-Condon factors calculated with the molecular parameters obtained at the same level. In particular, the observed methyl torsional progression could be reproduced very well through quantum-mechanical calculations using the molecular parameters obtained at this level. Dramatic lowering of the torsional barrier inferred from the experimental data was entirely compatible with the B3LYP6-311++G(3df,3pd) results. The torsional barrier and the internal rotational constant determined by fits to six torsional peaks were 53.6 and 5.20 cm(-1), respectively. A brief discussion at the level of molecular orbital is presented to account for the dramatic lowering of the torsional barrier upon ionization.

One-photon mass-analyzed threshold ionization spectroscopy of CH2BrI: Extensive bending progression, reduced steric effect, and spin-orbit effect in the cation

One-photon mass-analyzed threshold ionization (MATI) spectrum of CH2BrI was obtained using coherent vacuum-ultraviolet radiation generated by four-wave difference-frequency mixing in Kr. Unlike CH2ClI investigated previously, a very extensive bending (Br-C-I) progression was observed. Vibrational frequencies of CH2BrI+ were measured from the spectra and the vibrational assignments were made by utilizing frequencies calculated by the density-functional-theory (DFT) method using relativistic effective core potentials with and without the spin-orbit terms. A noticeable spin-orbit effect on the vibrational frequencies was observed from the DFT calculations, even though its influence was not so dramatic as in CH2ClI+. A simple explanation based on the bonding characteristics of the molecular orbitals involved in the ionization is presented to account for the above differences between the MATI spectra of CH2BrI and CH2ClI. The 0-0 band of the CH2BrI spectrum could be identified through the use of combined data from calculations and experiments. The adiabatic ionization energy determined from the position of this band was 9.5944+/-0.0006 eV, which was significantly smaller than the vertical ionization energy reported previously.

Vibrational assignment and Franck-Condon analysis of the mass-analyzed threshold ionization (MATI) spectrum of CH2ClI: the effect of strong spin-orbit interaction

Detailed analysis of the one-photon mass-analyzed threshold ionization (MATI) spectrum of CH(2)ClI is presented. This includes the determination of the ionization energy of CH(2)ClI, complete vibrational assignments, and quantum-chemical calculations at the spin-orbit density-functional-theory (SODFT) level with various basis sets. Relativistic effective core potentials with effective spin-orbit operators can be used in SODFT calculations to treat the spin-orbit term on an equal footing with other relativistic effects and electron correlations. The comparison of calculated and experimental vibrational frequencies indicate that the spin-orbit effects are essential for the reasonable description of the CH(2)ClI(+) cation. Geometrical parameters and thus the molecular shape of the cation are greatly influenced by the spin-orbit effects even for the ground state. Calculated geometrical parameters deviate substantially for different basis sets or effective core potentials. In an effort to derive the exact geometrical parameters for this cation, SODFT geometries were further improved utilizing Franck-Condon fit of the MATI spectral pattern. This empirical fitting produced the well-converged set of geometrical parameters that are quite insensitive to the choice of SODFT calculations. The C-I bond length and the Cl-C-I bond angle show large deviations among different SODFT calculations, but the empirical spectral fitting yields 2.191 +/- 0.003 Angstroms for the C-I bond length and 107.09 +/- 0.09 degrees for the Cl-C-I angle. Those fitted geometrical parameters along with the experimental vibrational frequencies could serve as a useful reference in calibrating relativistic quantum-chemical methods for radicals.

A high-resolution pulsed field ionization-photoelectron-photoion coincidence study of vinyl bromide

By employing the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method, we have examined the unimolecular dissociation reaction of energy-selected C(2)H(3)Br(+) to form C(2)H(3) (+)+Br near its threshold. The analysis of the breakdown curves for C(2)H(3)Br(+) and C(2)H(3) (+) yields a value of 11.9010+/-0.0015 eV for the 0 K dissociative photoionization threshold or appearance energy (AE) for C(2)H(3) (+) from C(2)H(3)Br. This AE(C(2)H(3) (+)) value, together with the ionization energy (IE) for C(2)H(3)Br (9.8200+/-0.0015 eV) obtained by PFI-PE and threshold photoelectron (TPE) measurements, has allowed the determination of the 0 K dissociation energy (D(0)) for the C(2)H(3) (+)-Br bond to be 2.081+/-0.002 eV. The 0 K AE(C(2)H(3) (+)) from C(2)H(3)Br obtained in this study corresponds to DeltaH(f0) ( composite function )(C(2)H(3) (+))=1123.7+/-1.9 kJ/mol. Combining the latter value and the known DeltaH(f0) ( composite function )(C(2)H(3))=306.7+/-2.1 kJ/mol, we calculated a value of 8.468+/-0.029 eV for the IE(C(2)H(3)), which is in accord with the result obtained in the previous photoionization efficiency study. We have also carried out high-level ab initio calculations for the IE(C(2)H(3)) at the Gaussian-3 and the CCSD(T,full)/CBS level of theory. The CCSD(T,full)/CBS prediction of 8.487 eV for the IE(C(2)H(3)-->bridged-C(2)H(3) (+)) is in good agreement with the IE(C(2)H(3)) value derived in the present experiment. Combining the 0 K AE(C(2)H(3) (+))=11.9010+/-0.0015 eV and the IE(C(2)H(3))=8.468+/-0.029 eV yields the value of 3.433+/-0.029 eV for D(0)(C(2)H(3)-Br). We have also recorded the TPE spectrum of C(2)H(3)Br in the energy range of 9.80-12.20 eV. Members (n=5-14) of four autoionizing Rydberg series converging to the C(2)H(3)Br(+)(A (2)A(')) state are observed in the TPE spectrum. The analysis of the converging limit of these Rydberg series and the vibrational TPE bands for C(2)H(3)Br(+)(A (2)A(')) has provided more precise values for the nu(6) (+) (1217+/-10 cm(-1)) and nu(8) (+) (478+/-8 cm(-1)) modes and the IE (10.9156+/-0.0010 eV) for the formation of C(2)H(3)Br(+)(A (2)A(')) from C(2)H(3)Br.

One-photon mass-analyzed threshold ionization spectroscopy of 1,3,5-trifluorobenzene: The Jahn-Teller effect and vibrational analysis for the molecular cation in the ground electronic state

One-photon mass-analyzed threshold ionization spectrum of 1,3,5-trifluorobenzene was obtained by using vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The Jahn-Teller parameters for the e' modes (nu(8)-nu(14)) of 1,3,5-C(6)H(3)F(3)(+) in the ground electronic state needed for spectral analysis were taken from the density functional theory results initially and were upgraded through fits to the experimental results. Excellent agreement was achieved between the experimental and calculated Jahn-Teller energy levels. Assignments of the Jahn-Teller inactive modes were accomplished by referring to the calculated frequencies and the selection rule. The ionization energy of 1,3,5-trifluorobenzene determined from the position of the 0-0 band was 9.6359+/-0.0006 eV.