Highly excited vibrational states

Abstract. The development of modern spectroscopic techniques and efficient computational methods have allowed a detailed investigation of highly excited vibrational states of small polyatomic molecules. As excitation energy increases, molecular motion becomes chaotic and nonlinear techniques can be applied to their analysis. The corresponding spectra get also complicated, but some interesting low resolution features can be understood simply in terms of classical periodic motions. In this chapter we describe some techniques to systematically construct quantum wave functions localized on specific periodic orbits, and analyze their main characteristics. 

Benjamin, I., Buch, V., Gerber, R. B., and Levine, R. D. (1984), Spacings Distribution for Highly Excited Vibrational States Role of Dynamial Symmetry, Chem. Phys. Lett. 107,515. 

When formaldehyde is subjected to suitable optical excitation it dissociates into H2 and CO. The process is thought to involve an excitation to the first excited singlet state followed by internal conversion to a highly excited vibrational state of the ground singlet state that dissociates according to the equation... 

The Montroll-Shuler equation can also predict how fast a molecule which is created in a highly excited vibrational state will decay to the equilibrium state. This is of interest in connection with chemiluminescence phenomena. In certain cases one finds experimentally that this relaxation is much faster than what one would expect from the master equation of Montroll and Shuler and improved versions of this equation. One possible mechanism for this fast relaxation is that although most of the collisions in which the diatomic molecule participates are between the diatomic molecule and an inert gas atom, there will also be some collisions between diatomic molecules. In the latter case we have the situation where two diatomic molecules in quantum state n collide producing, with fairly high probability, molecules in quantum states n I and n + 1, respectively. The number of such collisions is, of course quite small compared to the number of collisions of the first kind, but since they are so extremely efficient they may still be of importance. This mechanism, we believe, was first suggested in connection with chemiluminescence by Norrish in a Faraday Society discussion.5 The equations describing this relaxation had, however, been discussed several years earlier by Shuler6 and Osipov.7... 

We first briefly discuss in Section 13.1 the one-dimensional case and a simple two-dimensional model. The photodissociation of highly excited vibrational states of H2O and HOD, for which both experimental and theoretical results are available, will be reviewed in more detail in Sections 13.2 and 13.3. 

Let us note that the electron matrix element should be calculated for the equilibrium configuration of the nuclei of the initial molecule only in the case when the low vibrational states are occupied. At sufficiently high temperatures, when the highly excited vibrational states are occupied with noticeable probability, the electron matrix element must be calculated at an intermediate distance between the nuclei ... 

Mills IM (1992) Understanding spectra of highly excited vibrational states. In Murray I, Cowe lA (eds) Making Light Work Advances in Near Infrared Spectroscopy. VCH, Weinheim Mink J, Keresztury G (1988) Croat Chem Acta 61 731... 

The other important channel in the decay of the Ca-HX complex, ground state of the product CaCl, can be observed by laser-induced fluorescence and a very high vibrational excitation is detected. This high vibrational excitation arises from the sudden release at 4 A of the ground-state CaCl molecule, and thereby in a highly excited vibrational state, the distribution, maximum at u = 30, extends to v = 60. The resultant energy distribution has been interpreted with the use of the DIPR-DIP model [246]. 

Classical, Semiclassical, and Quantum Dynamics of Long-Lived Highly Excited Vibrational States of IVIatoms... 

It is also the case that measurements of gas-phase molecular distributions are used to analyze the products of recombinative desorption. In this case, as indicated in Fig. 16, the angular distribution is observed to peak along the normal to the surface, being proportional to cos"6 with n>2 and sometimes up to 12. Highly excited vibrational states are often found (Kubiak et al. 1984 Brown and Bernasek 1985 Mantell et al. 1986 Karikorpi et al. 1987 Harris et al. 1988b). We will have much more to say about such distributions in Section VI. The review by Comsa and David (1985) deals in detail with this subject. 

The observation that the reaction requires an induction time of tens of picoseconds can be used to differentiate between proposed mechanisms of how shock wave energy localizes to cause chemical reaction. This induction time is expected for mechanisms that involve vibrational energy transfer, such as multiphonon up-pumping [107], where the shock wave excites low frequency phonons that multiply annihilate to excite the higher frequency modes involved in dissociation. It is also consistent with electronic excitation relaxing into highly excited vibrational states before dissociation, and experiments are underway to search for electronic excitations. On the other hand, prompt mechanisms, such as direct high frequency vibrational excitation by the shock wave, or direct electronic excitation and prompt excited state dissociation, should occur on sub-picosecond time scales, in contrast to the data presented here. 

Considerable experimental effort has been aimed at elucidating the collision-free unimolecular dynamics of excited molecules. Processes of interest include the dynamics of highly excited vibrational states, which have been reached by multiphoton absorption, and the various electronic relaxation processes that can occur in electronically excited states of moderate to large molecules, etc. The idealized collision-free limit is approached either by extrapolating data to the limit of zero pressure or by performing experiments in molecular beams. Alternatively, estimates of expected collisional effects are made by using collision cross-sections that are computed from hard-sphere collision rates. These estimates are then utilized to determine whether the experiments are performed in the collision-free domain. 

W exchange along the asymmetric mode is several orders of magnitude faster than that along symmetric modes, which intensify the population of highly excited vibrational states in this type of vibration. 

X n Transition of CF starting from Highly Excited Vibrational States, J. Mol. Spectrosc. 205 (2001) 341-343. 

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