Vibrationally-rotationally cold

The power and beauty of jet techniques is the ability to produce a beam of molecules that are vibrationally and rotationally cold but are still vapors. These advantages of supersonic expansions, however, rely on the possibility of producing sufficient partial vapor pressure of the molecules of interest, typically a few millibars. By regulating the temperature of the beam, it is possible to vary the vapor pressure of the sample to the required values. As an example, hquid molecules, such as aromatic alcohols, have to be heated at about 80 °C to obtain the right vapor concentration in the beam, while other largely volatile molecules, such as aliphatic amines, must be kept in a cold bath around 0 °C to avoid saturation. 

Townsend et al. were able to further resolve the photodissociation of formaldehyde to show that the rotationally highly excited CO was accompanied by vibrationally cold H2, as expected for a reaction over a single-concerted TS (shown in Figure 8.21 as 95). A smaller component, observed at higher photon energies, involves production of rotationally cold CO and vibrationally hot H2. A PES created by fitting energies, computed at CCSD(T)/aug-cc-pVTZ, for... 

Real-time experiments (Khundkar and Zewail, 1990 Zewail, 1991) with a subpicosecond resolution have probed the unimolecular dynamics of NO2 NO + O (Ionov et al., 1993a) and H + CO2 HOCO -> HO 4- CO (Scherer et al., 1987, 1990 Ionov et al., 1993b). The NO2 experiment is described and discussed in section 6.2.3.1 (p. 196). The H + CO2 reaction and ensuing formation of HOCO is initiated by photodissociation of HI in the HI—CO2 van der Waals complex (Fig. 8.8). A subpicosecond laser pulse is used to initiate the reaction while a second laser pulse probes the product formation. The reactants are vibrationally and rotationally cold prior to excitation, and the experiments demonstrate that the H + CO2 reaction proceeds... 

Experimental product rotational and translational energy distributions derived from energy-selected dissociation reactions can frequently be characterized by a temperature which implies that the distribution is a canonical one. This is found even when rotationally cold reactants are prepared in a state-selective manner. How can this be We illustrate the origin of these canonical distributions by calculating the rotational and vibrational distributions for a system of classical harmonic oscillators. 

To facilitate comparison with existing 210 kcal mol" rotationally cold data for these patterns, rotationally hot trajectories were nm with the same total energy. To achieve this, angular momentum was sampled from a thermal distribution at 6000 K, and the remaining available energy was apportioned to vibration by scaling. 

The laser spectroscopic techniques provide much more detailed information about the state-dependent velocity distribution than measurements with mechanical velocity selectors. Note that in Fig. 4.13 not only Up(Na2) > t p(Na) but the velocity distribution of the Na2 molecules differs for different vibration-rotation levels (v, J). This is due to the fact that molecules are being formed by stabilizing collisions during the adiabatic expansion. Molecules in lower states have suffered more collisions with atoms of the cold bath. Their distribution n(v) becomes narrower and their most probable velocity Vp more closely approaches the flow velocity u. 

An elegant technique for studying van der Waals complexes at low temperatures was developed by Toennies and coworkers [442]. A beam of large He clusters (lO -lO He atoms) passes through a region with a sufficient vapor pressure of atoms or molecules. The He droplets pick up a molecule which either sticks to the surface or diffuses into the central part of the droplet, where it is cooled down to a low temperature of 100 mK up to a few Kelvin (see Fig. 4.22). Since the interaction with the He atoms is very small, the spectrum of this trapped molecule does not differ much from that of a free cold molecule. However, unlike cooling during the adiabatic expansion of a supersonic jet, where Tyib > Trot > Ttrans in this case Trot = Tyib = Thc [443 45]. This implies that all molecules are at their lowest vibration-rotational levels and the absorption spectrum becomes considerably simplified. 

Kotationally cold OCS molecules are ejected from a pulsed supersonic jet. The molecular beam is dissociated by a pulsed polarized laser beam at 222 nm. The internal states of the CO fragments are monitored by a circular polarized tunable VUV beam aligned perpendicularly to the dissociation beam. The VUV beam monitors the various vibrational rotational states through the CO s X A ir transition. It is shown that the CO photofragments are produced in the vibrationless ground state with a highly excited rotational distribution sharply peaked at J = 56. 

Generally, collision-induced quenching is far more efficient for rotation than vibration. Rotational quenching is driven by the angular anisotropy of the helium interaction with the molecule, and the timescale for a small impact parameter cold collision is similar to a rotational period. Vibrational relaxation, on the other hand, is driven by the dependence of the interaction potential on the internuclear separation in the... 

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