Molecule vibrational excitation

Ozone layer depletion, stratospheric, 21 525-529 Ozone level, reduction in, 21 528 Ozone molecules, vibrationally excited, 27 774... 

A molecule vibrationally excited by absorption of a laser photon can convert its excitation energy into translational (F - T transfer), rotational (F-> i ), vibrational (V- F) or even electronic energy (F- ) of the collision partners. 

Hot Band.—Long wavelength contribution to an absorption band arising from absorption of radiation by molecules vibrationally excited but in their ground electronic state [e.g., the transition S(i> = ()) -So( ff = l)]. 

The site dependence of the PES is more than just a variation of the barrier energy and distance from the surface, the early/lateness of the barrier and the curvature of the elbow PES can also change. This leads to a spatial separation of different processes, as noted above, for H2 on Cu surfaces, dissociation of the vibrational ground-state molecules is dominated by the bridge sites, while molecules vibrationally excite/de-excite largely at the atop site. In fact, site specificity of reactivity is also state specific [45]. Thus for H2/Cu(1 0 0), the bridge site is the most favoured for dissociation of vibrationally cold molecules, but the atop site is the most favoured for vibrationally excited molecules, as shown in Fig. 4. 

G.K. Paramonov and V.A. Savva, Resonance Effects in Molecule Vibrational Excitation by Picosecond Laser Pulses , Phys. Lett. A 97, 340 (1983). 

Much of the previous section dealt with two-level systems. Real molecules, however, are not two-level systems for many purposes there are only two electronic states that participate, but each of these electronic states has many states corresponding to different quantum levels for vibration and rotation. A coherent femtosecond pulse has a bandwidth which may span many vibrational levels when the pulse impinges on the molecule it excites a coherent superposition of all tliese vibrational states—a vibrational wavepacket. In this section we deal with excitation by one or two femtosecond optical pulses, as well as continuous wave excitation in section A 1.6.4 we will use the concepts developed here to understand nonlinear molecular electronic spectroscopy. 

An important further consequence of curvature of the interaction region and a late barrier is tliat molecules that fail to dissociate can return to the gas-phase in vibrational states different from the initial, as has been observed experunentally in the H2/CU system [53, ]. To undergo vibrational (de-)excitation, the molecules must round the elbow part way, but fail to go over the barrier, eitlier because it is too high, or because the combination of vibrational and translational motions is such that the molecule moves across rather than over the barrier. Such vibrational excitation and de-excitation constrains the PES in that we require the elbow to have high curvature. Dissociation is not necessary, however, for as we have pointed out, vibrational excitation is observed in the scattering of NO from Ag(l 11) [55]. 

The site specificity of reaction can also be a state-dependent site specificity, that is, molecules incident in different quantum states react more readily at different sites. This has recently been demonstrated by Kroes and co-workers for the Fl2/Cu(100) system [66]. Additionally, we can find reactivity dominated by certain sites, while inelastic collisions leading to changes in the rotational or vibrational states of the scattering molecules occur primarily at other sites. This spatial separation of the active site according to the change of state occurring (dissociation, vibrational excitation etc) is a very surface specific phenomenon. 

Faubel M and Toennies J P 1977 Scattering studies of rotational and vibrational excitation of molecules Adv. Atom. Mol. Phys. 13 229... 

Pibel C D, Sirota E, Brenner J and Dai H L 1998 Nanosecond time-resolved FTIR emission spectroscopy monitoring the energy distribution of highly vibrationally excited molecules during collisional deactivation J. Chem. Phys. 108 1297-300... 

Okamoto H, Nakabayashi T and Tasumi M 1997 Analysis of anti-Stokes RRS excitation profiles as a method for studying vibrationally excited molecules J. Phys. Chem. 101 3488-93... 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

Related results of promotion (catalysis) and inliibition of stereonuitation by vibrational excitation have also been obtained for the much larger molecule, aniline-NHD (CgH NHD), which shows short-time chirality and stereonuitation [104. 105]. This kind of study opens the way to a new look at kinetics, which shows coherent and mode-selective dynamics, even in the absence of coherent external fields. The possibility of enforcing coherent dynamics by fields ( coherent control ) is discussed in chapter A3.13. 

McCoy A B and Siebert E L 1996 Canonical Van VIeck pertubation theory and its applications to studies of highly vibrationally excited states of polyatomic molecules Dynemics of Moiecuies end Chemicei Reections ed R E Wyatt and J Z H Zhang (New York Dekker) p 151... 

An important area that has yet to be fully explored is the effect of the flexibility of water molecules. The intennolecular forces in water are large enough to cause significant distortions from the gas-phase monomer geometry. In addition, the flexibility is cmcial in any description of vibrational excitation in water. 

Actually, collisions in which tlie batli becomes vibrationally excited are relatively rare, occurring witli a typical probability of 1% per gas-kinetic collision [6, 8, H and 13]. More common are processes tliat produce rotational and translational excitation in tlie batli acceptor while leaving tlie molecule in its ground (vibrationless) OO O state. 

C3.3.4 DEDUCING ENERGY TRANSFER MECHANISMS FROM POPULATION AND VELOCITY DISTRIBUTIONS OF THE SCATTERED BATH MOLECULES ROTATIONAL STATE POPULATION DISTRIBUTIONS FOR VIBRATIONAL EXCITATION OF THE BATH... 

Figure C3.3.8. A typical trajectory for a soft collision between a hot pyrazine molecule and a CO2 bath molecule in which the CO 2 becomes vibrationally excited.

The probability distribution functions shown in figure C3.3.11 are limited to events that leave the bath molecule vibrationally unexcited. Nevertheless, we know that the vibrations of the bath molecule are excited, albeit with low probability in collisions of the type being considered here. Figure C3.3.12 shows how these P(E, E ) distribution... 

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