The structure of floppy molecules: the Rg·XH/D (Rg=Ar, Ne, and Kr, X=O or S) family of complexes

Near-IR spectrum of NO(X2Π)-Xe: a joint experimental-theoretical investigation

Employing the method of constant photon energy sum (CONPHOENERS) scans, we measure the near-IR spectrum of NO-Xe in the region of the first vibrational overtone of the NO monomer. Three bands are detected, which are assigned as the origin band located at 3722.60 cm(-1) and as bands with excitation of one quantum of z-axis rotation (3726.07 cm(-1)) and one quantum of bending vibration (3739.02 cm(-1)), respectively. The partially resolved rotational and electronic fine structures of the bands are analyzed with the help of a full quantum mechanical bound-state calculation using the ab initio potential energy surfaces of Kłos et al. (J. Chem. Phys. 2012, 137, 014312/1-014312/14). We perform a linear least-squares fit to the calculated energy levels to determine a set of spectroscopic constants that describe not only the overall rotation of the complex but also the electrostatic splitting due to the sum potential and the P-type doubling due to the difference potential. Using these results as guidance, we are able to simulate the experimental spectra. The comparison with the results from the theoretical treatment confirms the high quality of the ab initio treatment. The position of the excited bands is predicted with sub-wavenumber accuracy. Also, the rotational constants for all bands are found within less than 5%. Some differences are found for the amount of P-type doubling, which is overestimated by the theoretical treatment. Constants for the electrostatic splitting are in reasonable agreement for the origin band. Larger deviations are found for the vibrationally excited band, which points toward some inaccuracies in the potential energy surfaces.

Collisional Energy Transfer from Highly Vibrationally Excited Radicals Is Very Efficient

Although highly vibrationally excited (HVE) radicals are ubiquitous in natural environments, the effect of collisional energy transfer (ET) on their reactivity has yet to be fully characterized. We have used time-resolved IR emission spectroscopy to characterize the vibrational-to-translational quenching of a small HVE radical, ketenyl (HCCO), by inert gases. Photolysis of ethyl ethynyl ether at 193 nm provides HVE HCCO in the X̃(2)A″ electronic ground-state, with a nascent internal energy of 2.2 ± 0.6 eV. IR emission is monitored as an indicator of vibrational energy, and spectral modeling allows direct determination of the average energy lost per collision as a function of the internal energy. Collisional deactivation of HVE HCCO is shown to be minimally an order of magnitude more efficient than closed-shell molecules of comparable size. Schwartz-Slawsky-Herzfeld-Tanczos (SSHT) theory, modified for HVE molecules, suggests that this ET enhancement is due to a strong attractive intermolecular interaction.

The Ã-state dissociation continuum of NO-Ar and its near infrared spectrum

After preparing NO-Ar in a vibrational state correlating with the first overtone vibration in NO, we recorded its hot band UV spectrum by monitoring simultaneously the intensity in the NO(+) and the NO(+)-Ar ion channels. In this way, the bound as well as the continuous part of the electronic Ã←X̃ spectrum are observed directly. Below the dissociation threshold, the intensity is found exclusively in the NO(+)-Ar ion channel while above it is found in the NO fragment ion channel. We observe simultaneously intensity in both ion channels only for a very narrow frequency range near the dissociation threshold. Structures in the dissociation spectrum correlate well with the thresholds for production of NO(A) in different rotational states. At frequencies well above the dissociation threshold, NO-Ar is detected efficiently as a NO fragment. This fact has been exploited to record the near IR spectrum of NO-Ar with significantly increased sensitivity. The dissociation detected spectra are essentially identical to our previous constant photon energy sum (CONPHOENERS) scans [B. Wen, Y. Kim, H. Meyer, J. Kłos, and M. H. Alexander, J. Phys. Chem. A 112, 9483 (2008)]. Several hot band spectra have been remeasured with improved sensitivity enabling a comprehensive analysis yielding for the first time spectroscopic constants for levels associated with the potential surfaces of NO-Ar correlating with NO(v(NO) = 0 and 2). Since many NO-X complexes do not have a strong bound Ã-state spectrum, although they do have a Ã-state dissociation continuum, there is the possibility to record their near IR spectra by employing dissociation detection.

The structure of the NO(X (2)Pi)-N(2) complex: A joint experimental-theoretical study

We report the first measurement of the spectrum of the NO-N(2) complex in the region of the first vibrational NO overtone transition. The origin band of the complex is blueshifted by 0.30 cm(-1) from the corresponding NO monomer frequency. The observed spectrum consists of three bands assigned to the origin band, the excitation of one quantum of z-axis rotation and one associated hot band. The spacing of the bands and the rotational structure indicate a T-shaped vibrationally averaged structure with the NO molecule forming the top of the T. These findings are confirmed by high level ab initio calculations of the potential energy surfaces in planar symmetry. The deepest minimum is found for a T-shaped geometry on the A(")-surface. As a result the sum potential also has the global minimum for this structure. The different potential surfaces show several additional local minima at slightly higher energies indicating that the complex most likely will perform large amplitude motion even in its ground vibrational state. Nevertheless, as suggested by the measured spectra, the complex must, on average, spend a substantial amount of time near the T-shaped configuration.

The near IR spectrum of the NO(X(2)Pi)-CH4 complex

We report the first measurement of the near IR spectrum of the NO-CH(4) complex in the region of the first vibrational NO overtone transition in an IR-resonance enhanced multiphoton ionization double resonance experiment. The origin band is located at 3723.26 cm(-1), i.e., redshifted by 0.59 cm(-1) from the corresponding NO monomer frequency. The observed spectrum consists of two bands assigned to the origin band and the excitation of hindered rotation of the NO monomer in the complex similar to z-axis rotation. The spacing and the relative intensity of the bands are consistent with a structure in which NO resides preferentially in a position perpendicular to the intermolecular axis. The deviation from the linear configuration with C(3v) symmetry can be regarded as a Jahn-Teller (JT) distortion. Each band is dominated by two broad peaks with a few resolved rotational structures. The large spacing between the two peaks is indicative of significant angular momentum quenching, possibly another manifestation of the JT effect. The delay dependence between the IR and UV laser pulses reveals a lifetime of about 10 ns for the vibrationally excited complex due to vibrational predissociation. On the other hand, the linewidth of the narrowest spectral features indicates a much shorter excited state lifetime of about 100 ps, most likely due to intramolecular vibrational redistribution.

Joint experimental-theoretical investigation of the lower bound states of the NO(X2Pi)-Kr complex

We describe the first measurement of the near IR spectrum of the NO-Kr van der Waals complex. A variant of IR-REMPI double-resonance spectroscopy is employed in which the IR and UV lasers are scanned simultaneously in such a way that throughout the scan the sum of the two photon energies is kept constant, matching a UV resonance of the system. In the region of the first overtone vibration of the NO monomer, we observe several rotationally resolved bands for the NO-Kr complex. In addition to the origin band located at 3723.046 cm(-1), we observe excited as well as hot bands involving the excitation of one or two quanta of z-axis rotation. Another band is assigned to the excitation of one quantum of bending vibration. The experimental spectra are compared with results of bound-state calculations for a new set of potential energy surfaces calculated at the spin-restricted coupled cluster level. For the average vibration-rotation energies, there is excellent agreement between the theoretical results based on the coupled states (CS) approximation and the full close-coupling (CC) treatment. Finer details like the electrostatic splitting and the P-type doubling of the rotational levels are accounted for only within the CC formalism. The comparison of the CC results with the measured spectra confirms the high quality of the PESs. However, the high resolution of the experiments is sufficient to identify some inaccuracies in the difference between the potential energy surfaces of A' and A'' reflection symmetry.

Inelastic scattering of OH(X 2Π) with Ar and He: a combined polarization spectroscopy and quantum scattering study

One-colour polarization spectroscopy (PS) on the OH A (2)Sigma(+)- X (2)Pi(0,0) band has been used to measure the removal of bulk rotational angular momentum alignment of ground-state OH(X (2)Pi) in collisions with He and Ar. Pseudo-first-order PS signal decays at different collider partial pressures were used to determine second-order decay rate constants for the X (2)Pi(3/2), J = 1.5-6.5, e states. The PS signal decay rate constant, k(PS), is sensitive to all processes that remove population and destroy polarization. The contribution to k(PS) from pure (elastic) alignment depolarization within the initial level, k(DEP), can be extracted by subtracting the independently measured or predicted sum of the rate constants for total rotational energy transfer (RET), k(RET), and for Lambda-doublet changing, k(Lambda), collisions from k(PS). Literature values of k(RET) and k(Lambda) are available from experiments with He and Ar, and from quantum scattering calculations for Ar only. We therefore also present the results of new, exact, fully quantum mechanical calculations of k(RET) and k(Lambda) on the most recent ab initio OH(X)-He potential energy surface of Lee et al. [J. Chem. Phys. 2000, 113, 5736]. The results for k(DEP) from this subtraction for He are found to be modest, around 0.4 x 10(-10) cm(3) s(-1), whereas for Ar k(DEP) is found to range between 0.6 +/- 0.2 x 10(-10) cm(3) s(-1) and 1.7 +/- 0.3 x 10(-10) cm(3) s(-1), comparable to total population removal rate constants. The differences between k(DEP) for the two colliders are most likely explained by the presence of a substantially deeper attractive well for Ar than for He. The measurement of k(DEP) may provide a useful new tool that is more sensitive to the form of the long-range part of the intermolecular potential than rotational state-changing collisions.

Three-dimensional potential energy surface of the Ar–OH(Πi2) complex

Pure rotational transitions in the ground state for Ar-OH and Ar-OD [Y. Ohshima et al., J. Chem. Phys. 95, 7001 (1991) and Y. Endo et al., Faraday Discuss. 97, 341 (1994)], those in the excited states of the OH vibration, nu(s)=1 and 2, observed by Fourier-transform microwave spectroscopy in the present study, rotation-vibration transitions observed by infrared-ultraviolet double-resonance spectroscopy [K. M. Beck et al., Chem. Phys. Lett. 162, 203 (1989) and R. T. Bonn et al., J. Chem. Phys. 112, 4942 (2000)], and the P-level structure observed by stimulated emission pumping spectroscopy [M. T. Berry et al., Chem. Phys. Lett. 178, 301 (1991)] have been simultaneously analyzed to determine the potential energy surface of Ar-OH in the ground state. A Schrodinger equation, considering all the freedom of motions for an atom-diatom system in the Jacobi coordinate, R, theta, and r, was numerically solved to obtain energies of the rovibrational energy levels using the discrete variable representation method. A three-dimensional potential energy surface is determined by a least-squares fitting. In the analysis the potential parameters, obtained by ab initio calculations at the RCCSD(T) level of theory with a set of basis functions of aug-cc-pVTZ and midbond functions, are used as initial values. The determined intermolecular potential energy surface and its dependence on the OH monomer bond length are compared with those of an isovalent radical complex, Ar-SH.

Electronic spectroscopy of the deuterated isotopomers of the NO∙methane molecular complex

The molecular complexes formed between a nitric oxide molecule and the various deuterated isotopomers of the methane molecule have been studied in a supersonic jet expansion. The electronic spectrum arising from the transition corresponding to a 3s<--pi* excitation (approximately A (2)Sigma(+)<-- approximately X (2)Pi) located on the NO chromophore has been recorded employing resonance-enhanced multiphoton ionization spectroscopy, with each of CH(4), CH(3)D, CH(2)D(2), CHD(3), and CD(4) as the complexing partner. Rich spectra are obtained, whose appearance changes in a systematic way as the amount of deuteration increases. Unexpectedly, it was possible to record spectra not only in the parent mass channel, but also in various fragment channels; this also led to the identification of some O atom resonances; and their origin is discussed. Discussion is presented of the structure in the spectra, and its possible sources including hindered internal rotation of the methane and NO moieties, overall rotation of the complex, and tunneling. In addition, some guidance has been gleaned from ab initio calculations, and these are discussed in the light of the experimental results.

Bound states and scattering resonances of OH(A)–He

The OH-He complex has been observed using laser excitation of the A 2sigma+-X 2pi transition. The bands of the complex were close to the monomer rotational lines that terminate on the n = 0, 1, and 2 levels of OH(A). The unresolved band associated with He.OH (A,n=0) was redshifted from the OH parent line by 1.6 cm(-1), providing a direct measurement of D0'-D0". The complex features associated with n = 1 and 2 were identified as scattering resonances. They have been assigned by comparison with resonance structures derived from close-coupling calculations. The ab initio potential energy surface of H.-S. Lee, A.B. McCoy, R.R. Toczylowski, and S.M. Cybulski, [J. Chem. Phys. 113, 5736 (2000)] was used in these calculations. The level of agreement between the observed and predicted resonances indicated that the ab initio surface is reasonably accurate.

Spectroscopic and theoretical characterization of the AΔ2-XΠ2 transition of CH–Ne

The A2delta-X2pi transition of CH-Ne was examined using laser-induced fluorescence and fluorescence depletion techniques. The spectrum was found to be particularly congested due to the large number of bound states derived from the CH(A,n=2)+Ne interaction, and the small energy spacings between these states resulting from the relatively weak anisotropy of the van der Waals bond. High-level ab initio calculations were used to generate two-dimensional potential energy surfaces for CH(X)-Ne and CH(A)-Ne. The equilibrium structures from these surfaces were bent and linear for the X and A states, respectively. Variational calculations were used to predict the bound states supported by the ab initio surfaces. Empirical modification of the potential energy surfaces for the A state was used to obtain energy-level predictions that were in good agreement with the experimental results. Transitions to all of the optically accessible internal rotor states of CH(A,n=2)-Ne were identified, indicating that CH performs hindered internal rotations in the lowest-energy levels of the A and X states. The characteristics of the potential energy surfaces for CH-Ne in the X,A,B, and C states suggest that dispersion and exchange repulsion forces dominate the van der Waals interaction.

Spectroscopy of Ar–SH and Ar–SD. I. Observation of rotation-vibration transitions of a van der Waals mode by double-resonance spectroscopy

Rotation-vibration transitions of a van der Waals bending vibration, P = 1/2 <-- 3/2, of the Ar-SHSD (X 2pi) complexes in the electronic ground state have been observed by applying newly developed microwave-millimeter-wave double-resonance spectroscopy. The rotational energy-level structure for the two isotopomers, with hyperfine structure due to the hydrogen or deuterium nuclei and parity doublings in the P = 1/2 state, has now been clarified. Detailed explanation of the double-resonance technique is also given.

Spectroscopy of Ar–SH and Ar–SD. II. Determination of the three-dimensional intermolecular potential-energy surface

All the pure rotational transitions reported in the previous studies [J. Chem. Phys. 113, 10121 (2000); J. Mol. Spectrosc. 222, 22 (2003)] and newly observed rotation-vibration transitions, P = 1/2 <-- 3/2, for Ar-SH and Ar-SD [J. Chem. Phys. (2005), the preceding paper] have been simultaneously analyzed to determine a new intermolecular potential-energy surface of Ar-SH in the ground state. A Schrodinger equation considering the three-dimensional freedom of motion for an atom-diatom complex in the Jacobi coordinate, R, theta, and r, was numerically solved to obtain energies of the rovibrational levels using the discrete variable representation method. A three-dimensional potential-energy surface is determined by a least-squares fitting with initial values of the parameters for the potential obtained by ab initio calculations at the RCCSD(T)/aug-cc-pVTZ level of theory. The potential well reproduces all the observed data in the microwave and millimeter wave regions with parity doublings and hyperfine splittings. Several low-lying rovibrational energies are calculated using the new potential-energy surface. The dependence of the interaction energy between Ar and SH(2pi(i)) on the bond length of the SH monomer is discussed.

Molecular beam scattering of NO+Ne: A joint theoretical and experimental study

The collision dynamics of the NO+Ne system is investigated in a molecular beam scattering experiment at a collision energy of 1055 cm(-1). Employing resonance enhanced multiphoton ionization of NO, we measured state-resolved integral and differential cross sections for the excitation to various levels of both spin-orbit manifolds. The dependence of the scattered intensity on the laser polarization is used to extract differential quadrupole moments for the collision induced angular momentum alignment. The set of cross section data is compared with results of a full quantum mechanical close coupling calculation using the set of ab initio potential energy surfaces of Alexander et al. [J. Chem. Phys. 114, 5588 (2001)]. In previous work, it was found that the positions and rotational substructures for the lowest bend-stretch vibrational states derived from these surfaces agree very well with the observed spectrum of the NO-Ne complex. For the same potential, we find that the calculated cross sections show a less satisfactory agreement with the experimental data. While the overall Jf dependence and magnitude of the integral and differential cross sections are in good agreement, noticeable discrepancies exist for the angle dependence of the differential cross sections. In general, the calculated rotational rainbow structures are shifted towards larger scattering angles indicating that the anisotropy of the potential is overestimated in the fit to the ab initio points or in the ab initio calculation itself. For most states, we find the measured alignment moments to be in excellent agreement with the results of the calculation as well as with predictions of sudden models. Significant deviations from the sudden models are observed only for those fine-structure changing collisions which are dominated by forward scattering. Results of the full quantum calculation confirm the deviations for these states.