Vibrational predissociation spectroscopy

The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

While the use of direct absorption methods has grown, indirect action spectroscopic methods continue to be widely and successfully used in the study of neutral molecular clusters. As mentioned earlier, there are two commonly used detection methods, mass spectrometers and bolometers. Because of the variety of mass-spectroscopic methods, there is an equally wide range of techniques used in neutral cluster spectroscopy. One of the oldest among these involves electron-impact mass spectrometry of a cw neutral beam combined with vibrational predissociation spectroscopy using a tunable cw or pulsed laser. The advent of continuously tunable infrared sources (such as color center lasers and LiNbOa optical parametric oscillators) allowed for detailed studies of size and composition variation in neutral clusters. However, fragmentation of the clusters within the ionizer of the mass spectrometer, severely limited the identification of particular clusters with specific masses. Isotopic methods were able to mitigate some of the limitations, but only in a few cases. 

M. F. Vernon, D. J. Krajnovich, H. S. Kwok, J. M. Lisy, Y. R. Shen, and Y. T. Lee,/. Chem. Phys., 77,47 (1982). Infrared Vibrational Predissociation Spectroscopy of Water Clusters by the Crossed Laser-Molecular Beam Technique. 

Figure 7 The setup of the instrument used to measure infrared spectra of ionic complexes via vibrational predissociation spectroscopy.

A kind of more complicated metal oxide clusters, cerium oxide clusters are studied by Sierka s group. Ce202", Ce304 , and (Ce02)m CeO (m = 0-4) are unambiguously determined by a combination of GA-DFT method and infrared vibrational predissociation spectroscopy of the cluster-rare gas atom complexes. The most stable structures of even small gas-phase aggregates of cerium oxide with 2-5 cerium atoms show structural motifs reminiscent of the bulk ceria. This is different from main group and transition metal oxide clusters, which often display structural features that are distinctly different from the bulk structure. 

The splitting of spectral features is one of the most direct signatures for PT to occur [6]. However, such data is only available from high-resolution spectroscopy of small molecules in the gas phase [7-10]. Recently, high-resolution vibrational spectra have been recorded for protonated clusters. In 2003, Asmis et al. [3] reported the vibrational spectram of the shared proton in the protonated water dimer. Johnson et al. used vibrational predissociation spectroscopy to characterize the spectral signature of protonated molecular clusters [11,12). And in 2010, Duncan et al. [13] presented photodissociation IR spectra of water clusters (n = 2 — 5)... 

Cline, J.I., Reid, B.P., Evard, D.D., Sivakumar, N., Halberstadt, N., and Janda, K.C. (1988). State-to-state vibrational predissociation dynamics and spectroscopy of HeCl2 Experiment and theory, J. Chem. Phys. 89, 3535-3552. 

Schuder, M. D. and Nesbitt, D. J., High resolution near infrared spectroscopy ofHCl—DCl and DCl—HCl Relative binding energies, isomer conversion rates, and mode specific vibrational predissociation, J. Chem. Phys. 100,7250-7267 (1994). 

I.r. laser spectroscopy and quadrupole mass spectrometry were used by Fischer et al. to study vibrational predissociation of clusters of C2H4, and CsHg, but-l-ene, cis- and trans-but-2-ene, and isobutene. They obtained spectra in the range 2900—3200 cm and for C2H4 clusters predissociation was observed to result from excitation near the v-i, and vg fundamentals and the i 2 + V12 combination band. The vibrational bands were observed to have Lorentzian lineshapes with IWHM of ca. 5 cm. A homogeneous broadening mechanism was assumed and the widths were used to calculate excited-state lifetimes. Valentini and co-workers studied the predissociation of C2H4 clusters at 950 cm in a crossed laser/molecular beam apparatus. 

Cavity ring down (CRD) spectroscopy, having proven to be a very sensitive method for detecting molecular species in a wide variety of environments, has also been applied to the mid infrared vibrational spectroscopy of hydrogen-bonded clusters of water " and alcohols.As a direct absorption method, it can be used to quantitatively measure important molecular properties, such as absorption cross sections and coefficients. Knowing these properties, as a function of cluster size and structure, is useful in making the connection to the condensed phase. The sensitive detection of methanol clusters, as shown in Fig. 13, is of considerable importance. These particular measurements nicely complement the action spectra of methanol clusters, detected by depletion of mass-detected signal via vibrational predissociation. 

The supersonic jet molecular beam electronic spectroscopy experiments of Smalley, Levy and Wharton on He l2 were the first to reveal, from lineshape measurements, unambiguous vibrational predissociation lifetimes. 

Not only do the experimental vibrational predissociation lifetimes require interpretation, so do the increasingly sophisticated theoretical calculations whose results often fall out of a web of coupled differential equations or the convoluted algebra of quantum mechanics. In order to offer a qualitative overview of dynamical processes in van der Waals molecules, we shall introduce a selection rule which can provide insight into possible relaxation channels of vibrationally excited molecules. This selection rule concerns the change in a quantum number, Anj., which is to remain small for efficient vibrational predissociation processes. It bears a close analogy to the selection rules of optical spectroscopy which require small changes in quantum numbers Au, AJ, AS, etc. for efficient transitions between molecular states. Let us review the origin of the vibrational predissociation selection rule which has been developed in more detail elsewhere. ... 

ABSTRACT. Fluorescence excitation spectra of OH-Ar complexes are observed in the vicinity of several OH A - X 1X3/2 transitions. Intermolecular potentials derived from these spectra illustrate the dramatic change in the OH-Ar potential upon electronic excitation of the OH moiety. The OH (center-of-mass) to Ar distance is substantially reduced and Ar becomes much more tightly bound to OH. The differences in the OH-Ar potentials are also reflected in the rate of vibrational predissociation, which is at least a thousand times faster in the excited electronic state. Much smaller changes in the potentials are found upon vibrational excitation of OH, as shown in the ground electronic state by vibrational overtone spectroscopy. 

Vibrational excitation of the OH subunit introduces the possibility of vibrational predissociation of the OH-Ar complex as discussed above. Vibrational excitation of OH-Ar complexes also induces small changes in the intermolecular potentials which can be detected through detailed spectroscopic studies. We have performed two types of experiments to examine the influence of OH vibration on the intermolecular potentials -laser induced fluorescence and vibrational overtone spectroscopy. ... 

In principle these terms may be directly evaluated by vibronic spectroscopy, since diAcOi is just the "vibrational shift . For example, the frequency of mode 8 is 7 cm higher in pDFB-Ar than in pDFB. Then Eq. (11.8) gives AV o°), 8 ) = 2 x7 cm l = S.Ocm at the equilibrium value of r. Remarkably few assumptions are required to relate AcOi to the rate of vibrational predissociation of a van der Waals molecule with mode i excited.while the exact form of Eq. (I1.8) relies on the harmonic approximation... 

The linearly structure (HCN)2 cluster was studied by SCF and Cl calculations. Results available so far are within the framework of a collinear model only, given the fact that a full, reliable potential surface for this system is not available yet. We note that calculations of vibrational predissociation lifetimes that we carried out for this system in the framework of a preliminary collinear treatment have shown that to obtain lifetimes compatible with the experimental ones, the energy released from the excited C-H stretching mode must necessarily be mostly dumped into C-H bending and rotational excitation of the fragments. This means that the collinear treatment is unacceptable even qualitatively for describing the predissociation dynamics. However a collinear treatment may be a reasonable approximation for the spectroscopy of those modes that do not involve bending excitations. Table 1 shows the results of SCF and Cl calculations for several of the transition frequencies. Jacobi ("collision") coordinates seemed an intuitively reasonable choice for the SCF calculation, and were employed here. 

Van der Waals hetero-clusters are ideal model systems for the detailed study of a variety of photophysical and photochemical processes such as energy transfer, vibrational predissociation, fine-structure relaxation, and half-collision chemical reactions. In addition, the spectroscopy of these species provides central information on intermolecular forces and their additivity properties. 

Abstract In the present study, the effect of the potential energy surface representation on the infrared spectra features of the and Df clusters is investigated. For the spectral simulations, we adopted a recently proposed (Sanz-Sanz et al. in Phys Rev A 84 060502-1-4, 2011) two-dimensional adiabatic quantum model to describe the proton-transfer motion between the two H2 or D2 units. The reported calculations make use of a reliable on the fly DFT-based potential surface and the corresponding new dipole moment surface. The results of the vibrational predissociation dynamics are compared with earlier and recent experimental data available from mass-selected photodissociation spectroscopy, as well as with previous theoretical calculations based on an analytical ab initio parameterized surfaces. The role of the potential topology on the spectral features is studied, and general trends are discussed. 

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