Single-photon process

Multiphoton Absorption and Ionization. High laser powers can induce the simultaneous absorption of two or more photons that together provide the energy necessary to excite a transition this transition may be one that is forbidden as a single-photon process (8,297). Such absorption can be made Doppler-free by propagating two laser beams of frequency V in opposite directions, so the Doppler shifts cancel and a two-photon transition occurs at 2v for any absorber velocity. The signal is strong because aU absorbers contribute, and peak ampHtudes are enhanced by, which may... 

The transition to the C B- state of H 0 was achieved by a two photon absorption of KrF laser light near 248 nm (32). The OH(A-X) fluorescence excitation spectrum in the 247.9-248.5 nm range follows the rotational structure of the C B -+ X A transition. However, (i) the OH(A-X) fluorescence spectrum produced by the two photon dissociation of H 0 has a maximum population at N = 14, while single photon absorption near 124 nm generates OH fluorescence spectrum with a maximum population at N = 20 (ii) only absorption to Ka= 1 (and not Ka= 0 where K is the rotational angular momentum about the a axis) of the ClB-L state predissociates into 0H(a2%) + H probably through the B A state. Apparently, the two-photon absorption near 248 nm predominantly populates the c b state, while the single photon process populates the B A near 124 nm. 

When the pump laser wavelength is changed to 248 nm (5.0 eV), neutral CO desorption is observed but there is no desorption of ion species. The fluence dependence of the desorption yield for neutral CO at X = 248 nm also indicates a single-photon process. The rotational, translational, and vibrational... 

The thermal or photochemical decomposition of volatile dialkyl telluriums is of interest as a source of tellurium atoms for the production of semiconductor devices. The bond breaking enthalpy for the Te—C bond in dimethyl tellurium is 212.1 + 5.4 kJ mol , and in divinyl tellurium 226.8 +9.6kJmor . Dimethyl tellurium begins to decompose at 350° with formation of tellurium, methane, and other hydrocarbons Diethyl ditel lurium begins to decompose at an appreciable rate above 420° Photolysis of dimethyl tellurium and diethyl tellurium with 248 nm photons produced tellurium atoms in a single photon process. The liberated tellurium atoms were in the 5p 2 state. ... 

B are forbidden as single-photon processes, but allowed under two-photon selection rules. This mechanism is represented by the time-ordered diagram of Fig. 5(a), and is governed by the energy conservation relations of Eqs (4.2) and (4.3). 

Infrared Absorption is a single-photon process. Here, also, kiR = K 0 applies. Thus, infrared absorption detects only phonons at the F point of the first BZ. In this case, we have oo = L2, where ho) is the quantum energy of the infrared radiation. The frequencies or the wavenumbers of the optical phonons in molecular crystals are of the order of 3 THz or 100 cm" thus the wavelengths of infrared absorption are of the order of 100 /xm. Infrared spectroscopy of phonons in molecular crystals is therefore in fact far-infrared spectroscopy. The symmetry selection rules are complementary to those for Raman scattering for vibrations with u and g states w g transitions are allowed and g g transitions are forbidden. 

If the saturation condition is met, the velocity-tuned three-photon processes bum holes in the velocity profile of molecules at one third the velocities Uq at which normal single-photon processes can create holes. Freund et aL studied these effects in CH3F. The setup was similar to an earlier experiment carried out by the same group (see above. Ref. 215), and made use of a CO2 laser with the absorption cell installed inside the cavity. Lamb dips were detected as sharp variations of the output power of the laser. 

Figure 12 shows the optical drift due to the guiding laser. The input laser beam was again chopped and the output intensity was measured by the lock-in amplifier with and without an N2 gas environment after the optical and electrical packaging. The stability of the device related to the 1.3-/u,m wavelength has been measured with time. Initially, the device was in an Nj environment to prevent photo-oxidation [17] for 2 h, and then the Nj gas was removed, as shown in Fig. 12a. For the first 2 h the output is stable. After the N2 gas is removed and the device is exposed to the ambient atmosphere, the output intensity decreases to 0.7 of the initial value after 11 h. After that, however, the output intensity increases to about twice the initial value. This phenomenon has not been observed before. The decay rates R depend linearly on the input intensities. For an input intensity (/q) of 10 W/cm, the is 0.2 dB/h, and for 5/q the R is 1.06 dB/h as shown in Fig. 12. This linear optical power loss mechanism may be explained by a single-photon process [23]. On the other hand, the increase rates R do not depend linearly on the input intensities. For input intensities of/ and S/q, the rates R are 1 and 0.12 dB/h, respectively. Since depends on (intensity), the recovery mechanism could be related to a two-photon process [24]. Table 3 summarizes the R and...

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