Cold excited state

The techniques outlined above provide information on the structure and accessibility of the photochemical reaction paths. As mentioned, this information is structural (i.e., nondynamical) and provides insight into the mechanism of photoproduct formation from vibrationally cold excited state reactants such as those encountered in many experiments where slow excited state motion or/and thermal equilibration is possible (in cool jets, in cold matrices, and in solution). 

Altliough an MOT functions as a versatile and robust reaction cell for studying cold collisions, light frequencies must tune close to atomic transitions and an appreciable steady-state fraction of tire atoms remain excited. Excited-state trap-loss collisions and photon-induced repulsion limit achievable densities. 

A far-off resonance trap (FORT), in contrast, uses tire dipole force ratlier tlian tire spontaneous force to confine atoms and can therefore operate far from resonance witli negligible population of excited states. A hybrid MOT/dipole-force trap was used by a NIST-Maryland collaboration [26] to study cold collisions, and a FORT was... 

Figure 4. He (23S) + He laboratory angular distributions at same energy for (A) cold excited beam and (B) cold ground-center-of-mass state beam. Relative intensities differ because of velocity dependence of intensity transformation Jacobian.

Sonobe and Rosenfeld (48,49) have measured the 4.7 pm infrared emission of CO. The extent of the CO vibrational excitation can be estimated using a cold gas filter containing CO and a 4.7 pm filter. If CO is vibrationally excited there is a smaller amount of attenuation of 4.7 pm fluorescence by the cold gas filter. When ketene is photolyzed at 193 nm, they estimate from their data that the rotational and vibrational temperatures are about 6700 and 3700 K, respectively. A high rotational temperature suggests that the C-C-0 angle is bent in the excited state. The CO vibrational excitation becomes less for longer excitation wavelengths. 

However, the MEP may be a convenient measure of the progress of a molecule in a reaction, because in general a molecule will move, on average, along the MEP in a well-defined valley, and it is a good approximation of the motion of vibrationally cold systems (e.g., for photochemical reactions in which the excited state reactant has a small/controlled amount of vibrational excess energy). 

The study of reactive excited states of van der Waals complexes is the link between the laser-assisted collision and the photodissociation approach it brings the collisional problem into a much simpler photodissociation problem. Here, a cold complex which has a defined geometry is formed between the collision partners and optically excited to trigger the reactive process. This creates the photodissociation of a molecule with very weakly bound ground state. The van der Waals spectroscopy has already allowed the accurate determination of the interatomic potential [Na-Ar (Smalley et al. 1977 Tellinghuisen et al. 1979), HgAr (Breckenridge et al. 1985, 1994 Fuke et al. 1984)]. More complex collisional... 

The general result for naphthol and phenol clustered with up to 30 to 40 water molecules is that excited state proton transfer does not occur in cold clusters (Knochenmuss et al. 1993 Knochenmuss and Smith 1994). Proton transfer occurs in clusters only if the solvent is a good proton acceptor and if ions generated in... 

The spectra can further be interpreted to describe the excited-state dynamics [28b, d The advantage of our cold molecular jet experiment is that inhomogeneous effects are minimized. In absorption, all lines are lifetime broadened because of the efficient photochemistry of OCIO. The magnitude of this broadening does not depend on the rotational quantum numbers, but is strongly dependent on the... 

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