Spectroscopy collisional energy transfer

The difference between the two spectroscopic approaches stems from the means by which the excited state is populated. In fluorescence, absorption of radiation at the proper frequency is used for excitation in emission spectroscopy, collisional energy transfer with other atoms, molecules, ions or electrons produces the excited-state population. 

COLLISIONAL ENERGY-TRANSFER SPECTROSCOPY WITH LASER-EXCITED ATOMS IN CROSSED ATOM BEAMS A NEW METHOD FOR INVESTIGATING THE QUENCHING OF ELECTRONICALLY EXCITED ATOMS BY MOLECULES... 

The results obtained so far with collisional energy-transfer spectroscopy are restricted to excited sodium atoms A = Na(32/,3/2) and quenching by a variety of simple polar and nonpolar molecules. The technique is applicable to any vaporizable molecule and will be available for a number of other atoms as well in due course with the progress of laser technology. The E-V-R transfer processes from and to sodium atoms have a number... 

Collisional Energy-transfer Spectroscopy with Laser-excited Atoms in Crossed Atom Beams A New Method for Investigating the Quenching of Electronically Excited Atoms by Molecules... 

Radford (1961, 1962) and Radford and Broida (1962) presented a complete theory of the Zeeman effect for diatomic molecules that included perturbation effects. This led to a series of detailed investigations of the CN B2E+ (v — 0) A2II (v = 10) perturbation in which many of the techniques of modern high-resolution molecular spectroscopy and analysis were first demonstrated anticrossing spectroscopy (Radford and Broida, 1962, 1963), microwave optical double resonance (Evenson, et at, 1964), excited-state hyperfine structure with perturbations (Radford, 1964), effect of perturbations on radiative lifetimes and on inter-electronic-state collisional energy transfer (Radford and Broida, 1963). A similarly complete treatment of the effect of a magnetic field on the CO a,3E+ A1 perturbation complex is reported by Sykora and Vidal (1998). The AS = 0 selection rule for the Zeeman Hamiltonian leads to important differences between the CN B2E+ A2II and CO a/3E+ A1 perturbation plus Zeeman examples, primarily in the absence in the latter case of interference effects between the Zeeman and intramolecular perturbation terms. 

For the spectroscopy of vibrational-rotational transitions in molecules the laser-excited fluorescence is generally not the most sensitive tool, as was discussed at the end of Sect. 6.3.1. Optoacoustic spectroscopy, on the other hand, is based on collisional energy transfer and is therefore not applicable to molecular beams, where collisions are rare or even completely absent. For the infrared spectroscopy of molecules in a molecular beam therefore a new detection technique has been developed, which relies on the collision-free conditions in a beam and on the long radiative lifetimes of vibrational-rotational levels in the electronic ground state [6.64-6.66]. 

Both linear and nonlinear Raman spectroscopy can be combined with time-resolved detection techniques when pumping with short laser pulses [8.781. Since Raman spectroscopy allows the determination of molecular parameters from measurements of frequencies and populations of vibrational and rotational energy levels, time-resolved techniques give information on energy transfer between vibrational levels or on structural changes of short-lived intermediate species in chemical reactions. One example is the vibrational excitation of molecules in liquids and the collisional energy transfer from the excited vibrational modes into other levels or into translational energy of the collision partners. These processes proceed on picosecond to femtosecond time scales [8.77,8.79]. 

Molecular systems exist in discrete quantum states, the study of which lies in the realm of molecular structure and wave mechanics. Transitions between quantum states occur either by absorption or emission of radiation (spectroscopy) or by collisional processes. There are two main types of collisional transitions which are important in chemical physics these are first, reactive processes in which chemical rearrangement takes place (reaction kinetics), and secondly collisions in which the energy distribution is changed without overall chemical reaction. It may therefore be concluded that the energy transfer processes discussed here are of fundamental importance in all molecular systems, and that the subject, like molecular structure, is enormously varied and complex. 

The reactions of atoms and small molecules as studied by u.v., vacuum-U.V., and visible spectroscopy are discussed in a review which covers rate constants for addition and recombination reactions of molecules and atoms (includes Na, K, and Rb) collisional cross-sections rate constants for reactions and energy transfers of recently observed diatomic molecules and radicals (includes Lig). ... 

As shown by Fig. 14.15, the resonances occur near zero field, and it is easy to calculate the small Stark shifts with an accuracy greater than the linewidths of the collisional resonances. As a result it is straightforward to use the locations of the collisional resonances to determine the zero field energies of the p states relative to the energies of the s and d states. Since the energies of the ns and nd states have been measured by Doppler free, two photon spectroscopy,22 these resonant collision measurements for n = 27, 28, and 29 allow the same precision to be transferred to the np states. If we write the quantum defect dp of the K np states as... 

Abbreviations used in the tables calc = calculated value PT = photodetachment threshold using a lamp as a light source LPT = laser photodetachment threshold LPES = laser photoelectron spectroscopy DA = dissociative attachment attach = electron at-tachment/detachment equilibrium e-scat = electron scattering kinetic = dissociation kinetics Knud=Knudsen cell CT = charge transfer CD = collisional detachment and ZEKE = zero electron kinetic energy spectroscopy. 

In this chapter, we have dealt with radiative transfer of atoms and molecules between different energy states rather extensively. However, transitions can also be induced by collisions. Extensive information on the static and dynamic properties of atoms and molecules can be obtained from collision physics. Although this book is centred on the spectroscopy of atoms and molecules, the importance of collisional physics should be clearly pointed out. For studies of these aspects the reader is referred to [4.32-37]. 

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