Photodepletion spectra

The apparatus has been already described in Ref. [7]. The clusters are produced by laser vaporization of a sodium rod, with helium at about 5 bars as a carrier gas and a small amount of SF6. The repetition rate is 10 Hz. In this configuration, the vibrationnal temperature of the formed clusters is roughly 400 K,[10] that gives 85% of C2V geometry and 15% of C3V for a Boltzman distribution. The laser beams are focused onto the cluster beam between the first two plates of an axial Wiley Mac-Laren Time-Of-Flight mass spectrometer with a reflectron. The photoionization efficiency curve as well as the photoabsorption spectrum determined by a photodepletion experiement are displayed on Fig. 1(b) and 1(c) respectively. The ionization threshold is at 4.3 eV, close to the 4.4 eV calculated for the C3V isomer and 4.9 eV for the C2V isomer (see the Fig. 1 (b)). The conclusion arising out of the photodepletion spectrum shown on Fig 1(c) and from ab initio calculations of the excited states, [5] is that the observed... 

Figure 16. (a) Experimental photodepletion spectrum of the Na- FH complex and (b) calculated absorption spectrum assuming T = 250 K. Adapted from Ref [75]. 

Figure 24.6 displays the photodepletion spectrum of the Ba FCHs cluster in its excited A state. The complex is excited over the range 730-760 nm by laser pulses of nanosecond duration. The spectrum exhibits a structureless shape, consistent with a direct type of photodissociation. As shown in Figure 24.7, the laser excitation pumps the cluster into an excited repulsive state, which leads to Ba + FCH3 fragments. 

Very often the photodissociation spectrum reveals a clear structure, which may reflect the presence of different electronic and vibrational states of the transition state. A good example of this type of photodepletion spectrum is that for the Li FH van der Waals complex, shown in Figure 24.8. Here, the distinct peaks correspond to the presence of various electronic states and vibrational motions of the Li - FHp transition state, decaying into LiF - - H products this... 

Figure 24.6 Ba---FCH3 photodepletion spectrum, for excitation over the range 730-760 nm. Adapted from Gasmi et al, Eur. Phys. 3. D, 2005, 33 399, with permission of Springer Science and Business Media...

Figure 24.8 Photodepletion spectrum of the Li FH van der Waals complex. Both experimental and theoretical ab initio) results are compared, with a good agreement between them. Reproduced from Aguado et at, J. Chem. Phys., 2003,119 10086, with permission of the American Institute of Physics...

Figure 24.12 Ba - FCH3 photodepLetion spectrum in the range 547-630 nm. Notice the double-peak structure with an energy spacing of 150 cm. Reprinted with permission from Skowronek et ai, 3. Phys. Chem. 101 7468. Copyright 1997 American Chemical Society...

Third, one-photon excitation of the So (X) thiophosgene to the Si (A) state, followed by photodepletion of the S2 > So fluorescence (fluorescence depletion) by S (B, C) < So (A) excitation, by another photon, reveals the presence of the dark C state in the vicinity of the B state. The polarization as well as the inversion splitting patterns of the OODR-FD spectrum are consistent with the C state being the planar JB2 (na ) state predicted by an ab initio calculation. 

The spectrum of the Ba-FCHj complex has also been probed via photodepletion spectroscopy and it displays strong transitions in the region of the Ba Pi line with a 60 cross-section [259, 260]. This corresponds to the local excitation observed in the preceding section which results in the corresponding splitting (here doubling) of the p transitions. 

Fig. 1. (a) The two isomers of NsbF. (b) Photoionization efficiency curve of NasF. Ab initio vertical ionization energies of the isomers are indicated by vertical lines. The UV probe photon energy used in the photodepletion experiment, as well as the femtosecond pump+probe total energy, is indicated by down arrows, (c) Photoabsorption spectrum of NasF determined by photodepletion spectroscopy. The calculated oscillator strengths for vertical excitations are indicated for both the Cjy and Csv structures by straight and dotted lines respectiveIy.[S] The spectra of the femtosecond pulses are also indicated. 

The action spectra of these products can be obtained by monitoring the wavelength dependence of the Ba" " and BaF signals originating from the depletion of the parent complex. The action spectrum for the BaF reaction channel is displayed in the top trace of Figure 24.13 it was obtained by ionization with photons of wavelength 266 nm. A resemblance between the BaF and the Ba- -FCHa photodepletion signals is clearly manifested. 

Skowronek S, Pereira R, Gonzalez-Urena A. 1997. Spectroscopy and dynamics of excited harpooning reactions the photodepletion action spectrum of the Ba - FCHs complex . J. Phys. Chem. A 101(41) 7468-7475. 

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