Optical state selection

Na2 in the X ground state optical state selection and detection... 

In a beam the molecules can travel over considerable distances in the vacuum chamber without experiencing collisions. Depletion of the population of a specific level by means of optical pumping at one place can be probed at another place by observing the laser induced fluorescence. If the level is repopulated by some process in the region between the two places the laser induced fluorescence increases. This Rabi type experiment with optical state selection has been applied by Childs and Goodman for the detection of molecular hfs transitions in the radiofre-... 

Supermolecular spectra could perhaps be studied with state-selection using adequate molecular beam techniques. That would not be easy, however, because of the smallness of the dipole moments induced by in-termolecular interactions. For the purpose of this book, we will mostly deal with bulk spectra, or interaction-induced absorption of pure and mixed gases. A great variety of excellent measurements of such spectra exists for a broad range of temperatures, while state-selected supermolecular absorption beam data are virtually non-existent at this time. Furthermore, important applications in astrophysics, etc., are concerned precisely with the optical bulk properties of real gases and mixtures. 

Silvers S.J., Gottscho, R.A. and Field, R.W. (1981). Collisional depolarization of state selected (J,Mj) BaO A1S+ measured by optical-optical double resonance, J. Chem. Phys., 74, 6000-6008. 

Most of the basic ideas of atomic or molecular state selection by optical means were formulated near the advent of the laser (e.g. Kastler, 1950, and Dehmelt and Jefferts, 1962). However, widespread application of the technique only took place in the late 1970s and early 1980s with the availability of dependable tuneable CW and pulsed lasers. Only laser radiation carries a sufficiently high spectral density for significant manipulation of the thermal population of atomic (or molecular) levels in a beam. 

The second scheme is based on the state-selective reionization of the optically pumped neutral atomic beam, followed by ion counting [66]. The method has been used successfully to study long isotopic chains of the noble gases Ar [67],, 6Kr... 

Anderson et al. [34] have measured cross sections for CID of state-selected D2+ (v) + HD. Unfortunately, no data exists for H2+ (v) + H2 to compare with our work. We can, however, compare our results with those of Vance and Bailey [32], who obtained cross sections for an unselected beam of H2. If we assume their distribuiion of vibrational levels was Franck-Condon, we estimate an averaged cross section of 9.4 A2 at 8 eV. This compares favorably with their value of 10.9 A2 as well as with the value of 10.3 A2 measured by Lee et al. [36]. The increase of the CID cross sections with vibrational levels is not surprising, because the dissociation energy decreases rapidly with v. Nevertheless, it is impressive that CID cross sections as large as 7 A2 are obtained in this system. Simple theoretical treatments of CID, such as the optical model of Levine and Bernstein [38], predict that the cross sections should increase rapidly with translational energy. This is clearly not the case for H2+ + H2. 

Figure B2.3.8. Energy-level schemes describing various optical methods for state-selectively detecting chemical reaction products left-hand side, laser-induced fluorescence (LIF) centre, resonance-enhanced multiphoton ionization (REMPI) and right-hand side, coherent anti-Stokes Raman spectroscopy (CARS). The ionization continuum is denoted by a shaded area. The dashed lines indicate virtual electronic states. Straight arrows indicate coherent radiation, while a wavy arrow denotes spontaneous emission.

K. Bergmann, State selection via optical methods, in Atomic and Molecular Beam Methods, ed. by G. Scoles (Oxford Univ. Press, Oxford, 1988), p. 293... 

After a short time delay the fragment of interest is state-selectively ionized. For this, usually a second laser is used whose photon energy is tuned to a suitable resonance transition for REMPI. The expansion of the now ion spheres continues as before, since any excess kinetic energy is basically carried away by the photoelectron. However, the ion trajectories are affected when an electric field is applied, which is poled and tailored (ion optics) in such a way to accelerate the ions towards the field-free TOF tube. 

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