Vibrational Predissociation Spectroscopy Research Articles

Infrared spectra of the solvated hydronium ion: Vibrational predissociation spectroscopy of mass-selected H3O+.cntdot.(H2O)n.cntdot.(H2)m

Infrared spectra of the mass-selected clusters H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}{sm bullet}(H{sub 2}) and H{sub 3}O{sup +}{sm bullet}(H{sub 2}){sub n} (n = 1,2, and 3) were observed by vibrational predissociation spectroscopy. The clusters were mass-selected and then trapped in a radio frequency ion trap. Cluster dissociation by loss of H{sub 2} followed excitation of OH or H{sub 2} stretches. Spectra were recorded by detecting fragment ions as a function of laser frequency. From spectra of H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}{sm bullet}(H{sub 2}), the authors were able to determine the spectrum of the hydrated hydronium ion H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}, because the H{sub 2} formed weak complexes with the hydrates. Spectra in the OH stretching region (3,000-4,000 cm{sup {minus}1}) were observed at a resolution of 1.3 cm{sup {minus}1} for clusters n = 1,2, and 3. The structure of the clusters and the perturbing effect of the H{sub 2} were inferred from a comparison with recent unpublished ab initio vibrational frequencies calculated by Remington and Schaefer.

Vibrational predissociation spectroscopy of methanol dimers [(CH3OD)2 and (CH3OH)(CH3OD)] in the 9.6.mu.m region

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTVibrational predissociation spectroscopy of methanol dimers [(CH3OD)2 and (CH3OH)(CH3OD)] in the 9.6.mu.m regionJeffrey P. LaCosse and James M. LisyCite this: J. Phys. Chem. 1990, 94, 11, 4398–4400Publication Date (Print):May 1, 1990Publication History Published online1 May 2002Published inissue 1 May 1990https://doi.org/10.1021/j100374a008RIGHTS & PERMISSIONSArticle Views51Altmetric-Citations26LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (374 KB) Get e-Alerts

State-to-state vibrational predissociation dynamics and spectroscopy of HeCl2: Experiment and theory

The structure and vibrational predissociation dynamics of HeCl2 are studied by pump–probe spectroscopy and by three-dimensional quantum mechanical calculations. Parity selected excitation spectroscopy is used to confirm the essential features of the previous analysis of the HeCl2 B←X laser excited fluorescence spectra. Product vibrational and rotational state distributions are measured for the v′=6, 8, 12, 20, and 24 levels of HeCl2 in the B state. For the v′=6 and 8 levels the dependence of the product state distribution on the initially excited rotational state is also measured. Although the dissociation dynamics are dominated by Δv=−1, V→T energy transfer, several interesting effects are revealed by monitoring the product rotational degrees of freedom. Due to the symmetry of the HeCl2 potential, the parity of the initially excited HeCl2 rotational state is conserved during the dissociation dynamics. Even when a single initial rotational state is excited, the observed product rotational state distribution is bimodal. The product rotational distribution is nearly independent of the amount of kinetic energy released to the product degrees of freedom. Three-dimensional quantum mechanical calculations using a simple potential energy surface are remarkably successful at reproducing the details of the experimental measurements. Only five parameters of the potential were adjusted to calculate the excitation spectrum, the vibrational predissociation product state distributions, and the lifetimes of the excited states. Analysis of the dissociation mechanism in terms of simple models, however, is not straightforward. In particular, the impulsive, quasiclassical half-collision model is not compatible with the observed independence of the product rotational state distribution from the amount of kinetic energy which is released. The close agreement between state-to-state experiment and quantum theory on the HeCl2 dynamics shows that the shortcomings of more approximate theories are fundamental and cannot be attributed to lack of knowledge of the true potential energy surface.

Infrared spectroscopy of mass-selected Cs(CH3OH)+N N=4–16

The infrared spectra of mass-selected cesium cations solvated by 4–16 methanol molecules have been obtained from vibrational predissociation spectroscopy. The solvated ions were prepared in a molecular beam apparatus equipped with a quadrupole mass spectrometer. A cw CO2 laser operating in the 9.6 μ region was used to excite the methanol C–O stretch in the solvated ions. Depletion of the mass-selected solvated ion signal due to vibrational predissociation was used to generate the infrared absorption spectra. This work represents the first infrared study of metal ion–solvent clusters. The spectra display a clear variation with the number of solvents throughout the range of 4–16. A single peak at ∼1031 cm−1 steadily grew in intensity from four to ten methanol molecules. The presence of additional methanol molecules beyond ten did not affect the intensity of the 1030 cm−1 peak, but a new spectral feature became observable to higher frequency. The spectroscopic data are consistent with a first solvation shell size of ten methanol molecules. This is the first direct experimental observation of a filled solvent shell associated with the cesium cation. The C–O stretching frequencies of the methanol molecules are compared with existing experimental results involving neutral clusters.

Theory of vibrational predissociation spectroscopy of weak complexes

The density matrix formalism has been applied to study the vibrational predissociation spectroscopy of weak complexes or clusters. The spectral bandshapes and the nature of the bandwidths are critically examined.

Infrared spectroscopy of the cluster ions H+3⋅(H2)n

The vibrational spectra of the clusters H+3(H2)n were observed near 4000 cm−1 by vibrational predissociation spectroscopy. Spectra of mass-selected clusters were obtained by trapping the ions in a radio frequency ion trap, exciting vibrational transitions of the cluster ions to predissociating levels, and detecting the fragment ions with a mass spectrometer. Low resolution bands of the solvent H2 stretches were observed for the clusters of one to six H2 coordinated to an H+3 ion. The red shift of these vibrations relative to the monomer H2 frequency supported the model of H+9 as an H+3 with a complete inner solvation shell of three H2, one bound to each corner of the ion. Two additional bands of H+5 were observed, one assigned as the H+3 symmetric stretch, and the other as a combination or overtone band. High-resolution scans (0.5 and 0.08 cm−1) of H+n, n=5, 7, and 9 yielded no observable rotational structure, a result of either spectral congestion or rapid cluster dissociation. The band contour of the H+5 band changed upon cooling the internal degrees of freedom, but the peaks remained featureless. The observed frequencies of H+7 and H+9 agreed well with ab initio predictions, but those of H+5 did not. This deviation is discussed in terms of the large expected anharmonicity of the proton bound dimer H+5.

Vibrational predissociation spectroscopy of (HF)3

The vibrational predissociation spectra of (HF)3, (HF)2DF, and HF(DF)2 in the HF stretching region, and (DF)3 in the DF stretching region have been measured using a molecular beam apparatus equipped with a mass spectrometer and a LiNbO3 optical parametric oscillator. The structure of (HF)3 has been found to be cyclic with C3h symmetry. No evidence for a noncyclic complex was found. Following vibrational excitation of the HF stretch, there was direct evidence for the trimer vibrational predissociation to the dimer plus monomer and indirect evidence for complete predissociation to three monomers. The total translational energy in the dimer plus monomer channel was 30 cm−1. The inability to resolve rotational features in the vibrational band, combined with negligible broadening of the rotational band contour bracketed the upper state lifetime between 2.5 and 21 ps. The largest ion fragment of the trimer produced by electron impact ionization was the protonated monomer.

Vibrational overtone predissociation spectroscopy of hydrogen peroxide

Vibrational overtone predissociation spectroscopy, which detects the products of a unimolecular reaction initiated by overtone vibration excitation as a function of wavelength, is an informative means of studying highly vibrationally excited molecules. The spectrum, obtained by varying the excitation laser wavelength while keeping the probe laser tuned to interrogate a single product state, mirrors the overtone vibration absorption spectrum of those molecules that decompose into the detected quantum state. Using this technique to detect individual rotational states of the OH fragments from hydrogen peroxide excited in the regions of the third to the fifth OH stretching overtone (4νOH, 5νOH, 6νOH) reveals coarse and fine vibrational structure in the spectrum. A vibration-torsion model, in which the high frequency OH stretching vibration and the low frequency torsional vibration are separated adiabatically, recovers the observed transition wave numbers and intensities and predicts previously unobserved transitions. Because the measurements probe individual product quantum states, the excitation spectrum also depends on the partitioning of the products among their states. This appears as a variation in the intensity of different transitions with the product state that is probed, and a statistical model of the unimolecular decomposition in combination with the vibration-torsion model of the absorption spectrum explains the dependence of the overtone vibrational predissociation spectra on the probed rotational state of the OH product.

Infrared vibrational predissociation spectroscopy of (SF 6) mAr n clusters: Implications for effective laser isotope separation

Selective infrared vibrational predissociation (IRVP) of Van der Waals clusters in a free jet is shown to be an effective way to separate isotopes. Absolute separation factors α in excess of 1.1 are observed upon irradiation of a beam of SF6 diluted in argon with a 20 W cw CO2 laser. Selection of the wavelength permits enrichment or depletion of any one of the sulfur isotopes in the mixture. In order to understand the observed wavelength dependence of the isotope enrichment we have measured the effective IRVP spectra of the individual Van der Waals clusters (SF6)mArn with 1 ⩽ m ⩽ 3 and n ⩽ 9. A systematic study of these spectra reveals a significant red-shift which occurs upon attaching Ar atoms to (SF6)m. Such shifts can attain as much as 10 cm−1, as in the case between SF6 and SF6Ar9.

Infrared vibrational predissociation spectroscopy of (SF 6) mAr n clusters: Implications for effective laser isotope separation

Selective infrared vibrational predissociation (IRVP) of Van der Waals clusters in a free jet is shown to be an effective way to separate isotopes. Absolute separation factors α in excess of 1.1 are observed upon irradiation of a beam of SF 6 diluted in argon with a 20 W cw CO 2 laser. Selection of the wavelength permits enrichment or depletion of any one of the sulfur isotopes in the mixture. In order to understand the observed wavelength dependence of the isotope enrichment we have measured the effective IRVP spectra of the individual Van der Waals clusters (SF 6) mAr n with 1 ⩽ m ⩽ 3 and n ⩽ 9. A systematic study of these spectra reveals a significant red-shift which occurs upon attaching Ar atoms to (SF 6) m. Such shifts can attain as much as 10 cm −1, as in the case between SF 6 and SF 6Ar 9.

Vibrational predissociation spectroscopy of (C 2H 4) 2 and Ar—C 2H 4

The vibrational predissociation spectra of (C 2H 4) 2 and Ar—C 2H 4 have been measured using a molecular beam apparatus and a tunable infrared laser (an optical parametric oscillator), near the ν 9 (≈3100 cm −1) and ν 11 (≈2990 cm −1) modes of ethylene. The spectra are not homogeneously broadened and show considerable rotational structure at 0.9 cm −1 resolution. Analysis of the Ar—C 2H 4 spectra places the argon in the ethylene plane along the perpendicular bisector of the double bond at a distance of 3.6 Å.

Infrared vibrational predissociation spectroscopy of water clusters by the crossed laser-molecular beam technique

Water clusters formed in a molecular beam are predissociated by tunable, pulsed, infrared radiation in the frequency range 2900–3750 cm−1. Absorption spectra of the clusters are obtained by detecting the recoiling fragments off-axis from the molecular beam as a function of laser frequency using a rotatable mass spectrometer. By carefully adjusting the expansion conditions of the molecular beam and monitoring the largest cluster observable, excessive contamination by clusters larger than the specific one of interest is avoided. It is found that the spectra of clusters containing three or more water molecules absorb over the same frequency range as the liquid. Dynamical information on the predissociation process is obtained from the measured angular and velocity distributions of the fragments. An upper limit to the excited vibrational state lifetime of ∼1 μs is observed for the results reported here. The most probable dissociation process concentrates the available excess energy into the internal motions of the fragment molecules. From adiabatic dissociation trajectories and Monte Carlo simulations it is seen that the strong coupling present in the water polymers causes extensive energy sharing among the intermolecular motions in the polymer before dissociation, consistent with the experimentally measured energy distributions. Comparison between current intermolecular potentials describing liquid water and the observed frequencies is made in the normal mode approximation. The inability of any potential to predict the gross spectral features (the number of bands and their observed frequency shift from the gas phase monomer) suggests that substantial improvement in the potential energy functions is possible, but that more accurate methods of solving the vibrational wave equation are necessary before a proper explanation of the spectral fine structure is possible.