Absorption multiple-photon

Enolate anions (4e) that have been heated by infiared multiple photon absorption for which torsional motion about the H2C-C bond, which destabilizes the 7t orbital containing the extra electron, is the mode contributing most to vibration-to-electronic energy transfer and thus to ejection. 

The energy received from multiple photon absorption may also be used to activate and dissociate otherwise stable gaseous ions [133]... 

For radiation induced chemical reaction, a distinction is often made between single-photon and multiple-photon events. The differentiation is based on the intensity (flux) of the photon source. For single photon events, the maximum energy of mid-IR photons is ca. 2.4kj mole and near-IR photons ca. 48 kj mole [25, 26]. Therefore, single photon mid-IR irradiation is normally considered non-destructive. However, intense irradiation and hence multiple photon absorption in mid-IR is known to promote chemical transformations [27, 28]. As an example of NIR pro-... 

As large intensities are necessary to induce a nonlinear refractive index change, not only linear but also multiple photon absorption processes have to be considered. The intensity I and the induced nonlinear phase shift NL are coupled differential equations as a function of the propagation direction z. 

Multiple-photon absorption spectra of SF, NFI3, and C2H4 determined using a new optothermal detector... 

Xe + Mechanism of the formation of Xe via multiple-photon absorption of 50 ps, 530 nm laser pulses 988... 

The gas-phase photochemistry of haiogenated ethenes has been studied in the UV and VUV [60, 61], as well as in the infrared, using multiple-photon-absorption excitation with powerful CO2 laser sources [62-66]. Also, sensitized decompositions, for example using electronically excited Hg( P) atoms, have also been reported [67-69]. The net gas-phase photochemistry of these systems usually involves hydrogen halide elimination via three-and/or four-center transition states, with some evidence for simple bond fission producing halogen atoms in the case of Hgf Pj) photosensitization [70]. 

Laser isotope separation (LIS) utilizes small differences in the spectroscopic properties of isotopic substances. Each isotope-bearing substance absorbs a radiation of a particular wavelength. Separation of the excited species can be achieved by multiple-photon absorption or photopredissociation of molecules or chemical scavenging. 

Laser-induced reaction has been widely used to stimulate gas-surface interaction. Lasers are also used to probe molecular dynamics in heterogeneous systems as well. In the applied area, the laser photochemical techniques are successfully applied to produce well defined microstructures and new materials for microelectronic devices (1). Enhanced adsorption and chemical reaction on surfaces can be achieved by a photoexcitation of gaseous molecules, adsorbed species as well as solid substrates. The modes of the excitation include vibrational and electronic states of the gaseous species and of the adsorbates surface complexes. Both a single and a multiple photon absorption may be involved in the excitation process. 

Vibrationally excited processes can be applied in the formation of semiconductor films. One of the examples to demonstrate the photoenhanced chemisorption and reaction due to the vibrational excitation is the interaction of SF molecules with silicon (2). In this case, SFg molecules can be chemically activated by multiple photon absorption of CO2 laser either in the gas phase or in the adsorbed state. Deposition of Si on quartz or glass surface can also be stimulated by the decomposition of SiH enhanced by the irradiation of CO2 laser to the gas phase (3). 

A resonance ionization mass spectrometer (RIMS) uses a tunable, narrow bandwidth laser to excite an atom or molecule to a selected energy level that is then analyzed by MS. The selective ionization often is accomplished by absorption of more photons from the exciting laser, but can also be effected by a second laser or a broadband photon source. Multiple photon absorption can result in direct ionization or in production of excited species that can then be ionized with a low-energy photon source (IR laser) or by a strong electric field. Resonance ionization methods have been applied to nearly all elements in the periodic table and to many radionuclides, including Cs (Pibida et al., 2001), Th (Fearey et al., 1992), U (Herrmann et al., 1991), Np (Riegel et al., 1993), Pu (Smith, 2000 Trautmann et al., 2004 Wendt et al., 2000), radioxenon and radiokrypton (Watanabe et al., 2001 Wendt et al., 2000), and 41Ca (Wendt et al., 1999). 

In laser-induced gas-phase reactions the first step is absorption of a vibrational quantum which, at moderate pressures, would only heat the gas since V-T energy transfer occurs. If this process were dominant the laser would be no more than a fancy (and expensive) Bunsen burner. At low pressures conditions are more favorable for multiple-photon absorption. The photon flux is large enough so that more than one quantum can be absorbed before significant V-T transfer occurs. The only trick is to maintain resonance between the laser frequency and the various transitions to be excited. If only pure vibrational transitions were involved anharmonicity would ensure that a laser tuned to the fundamental frequency ( 0 1) would be nonresonant for other transitions. However, since there are also changes in rotational quantum number resonance may be reestablished. The resonance conditions are illustrated in Fig. 6.9. 

An example of multiple-photon absorption is found in the laser-induced decomposition of D3BPF3 irradiated with a continuous source... 

The rate of vibrational excitation of a molecule through multiple-photon absorption, Wexci depends on the radiation intensity I and the vibrational-transition cross sections ... 

Lokhman, V. N., Makarov, A. A., Petrova, I. Yu., Ryabov, E. A., and Letokhov, V. S. (1999). Transition spectra in the vibrational quasicontinuum of polyatomic molecules. IR multiple-photon absorption in SFe. II. Theoretical simulations and comparison with experiment. Journal of Physical Chemistry, 103, 11299-11309. 

---