Stimulated-emission pumping

In the A-type OODR scheme (Fig. 10.14c) the probe laser induces downward transitions from the upper level 2) of the pump transition to lower levels m). This process which is called stimulated-emission pumping may be regarded as a resonantly induced Raman-type transition. In case of monochromatic pump and probe lasers tuned to the frequencies Wj and W2 respectively, the resonance condition for a molecule moving with velocity v is 

A more thorough consideration starts similar to the discussion of saturation spectroscopy in Sect.7.2 from the absorption coefficient of the probe wave 

This represents a Doppler profile ao( ) with a narrow dip at the frequency 

If the second term in the bracket of (10.17) becomes larger than 1, 0(0 ) becomes negative, i.e. the probe wave is amplified instead of attenuated (Raman gain, see Sect. 5.7). 

Chopping of the pump laser and detection of the probe laser Raman signal behind a lock-in amplifier yields a narrow OODR signal with a line-width 

Note that the laser frequency co does not necessarily coincide with con = E2 — E )lh when a virtual level is reached by Li. In the case where o) = co 2 we have resonance-enhanced Raman transitions. 

For a collinear arrangement of the two laser beams k A 2 the Doppler shift becomes very small for k — A 2 A i, and the width of the OODR signal S o)2) is only determined by the sum of the level widths of the initial and final levels, while the width of the upper level 2) does not enter into the signal width. If the two levels 1) and m) are, for example, vibrational-rotational levels in the electronic ground state of a molecule, their spontaneous lifetimes are very long (even infinite for homonuclear diatomics ) and the level widths are only determined by the transit time of the molecule through the laser beams. In such cases, the linewidth of the OODR signal may become smaller than the natural linewidth of the optical transitions 11 2) (Sect. 9.4). 

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The resonance vector analysis has been used to explore all of the questions raised above on the fate of the polyad numbers in larger molecules, the most thoroughly investigated case so far probably being C2FI2- This molecule has been very extensively probed by absorption as well as stimulated emission pumping and dispersed fluorescence teclmiques [, 53, 70 and 71], the experimental spectra have been analysed in... 

Davis M J 1995 Trees from spectra generation, analysis, and energy transfer information Molecular Dynamics and Spectroscopy by Stimulated Emission Pumping ed H-L Dai and R W Field (Singapore World Scientific)... 

Stock C, Li X, Keller H-M, Schinke R and Temps F 1997 Unimolecular dissociation dynamics of highly vibrationally excited DCO x-A t- I- Investigation of dissociative resonance states by stimulated emission pumping spectroscopy J. Cham. Phys. 106 5333-58... 

Berry M T, Loomis R A, Gianoarlo L C and Lester M I 1991 Stimulated emission pumping of intermoleoular vibrations... 

The Dynamics of "Stretched Molecules" Experimental Studies of Highly Vibra-tionally Excited Molecules with Stimulated Emission Pumping. 

Northrup, F. J., and Sears, T. J. (1992), Stimulated Emission Pumping Applications to Highly Vibrationally Excited Transition Molecules, Ann. Rev. Phys. Chem. 43, 127. 

Dispersed Fluorescence and Stimulated Emission Pumping Spectroscopy Vibrational Assignment of the Feature States, J. Chem. Phys. 95, 6330. 

Yamanouchi, K., Takeuchi, S., and Tsuchiya, S. (1990), Vibrational Level Structure of Highly Excited S02 in the Electronic Ground State. II. Vibrational Assignment by Dispersed Fluorescence and Stimulated Emission Pumping Spectroscopy, J. Chem. Phys. 92, 4044. 

Figure 0.1 Stimulated emission pumping (SEP, Hamilton et al., 1986 Northrup and Sears, 1992) is a new experimental technique for accessing higher-lying vibrational levels of molecules in their ground electronic states. Shown is the SEP vibrational spectrum of S02, where a pair of dips represent one vibrational level. (Adapted from Yamanouchi, Takeuchi, and Tsuchiya, 1990.) The stick spectrum at the bottom represents the position of the vibrational levels given by Equation (0.1) with the constants given in Table 0.1. The bright levels are represented by longer sticks.

D. M. Jonas, X. Yang, and A. M. Wodtke. Axis-switching transitions and the stimulated emission pumping spectrum of HCN. J. Chem. Phys., 97(4) 2284-2298(1992). 

M. Silva, R. Jongma, R. W. Field, and A. M. Wodtke. The dynamics of stretched molecules experimental studies of highly vibrationally excited molecules with stimulated emission pumping. Annu. Rev. Phys. Chem., 52 811-852 (2001). 

Special issue, Molecular Spectroscopy and Dynamics by Stimulated-Emission Pumping, (J. Opt. Soc. Am. B 7, 1990). 

Stimulated emission pumping spectra of HCP X1 have been recorded via the A lA" [12] and C lA states [5]. These 0.05-cm-1 resolution spectra sample eigenstates rather than feature states, extending to Vib - 25,315 cm-1, above the X state zero-point level, about 1300 cm-1 above the ab initio predicted linear HPC saddle point [5, 13]. 

Experimentally one can investigate resonances by various spectroscopic schemes, as indicated in Fig. 1 by direct overtone pumping [11] from the ground vibrational state, by vibrationally mediated photodissociation [12] using an excited vibrational level as an intermediate, or by stimulated emission pumping (SEP) [13-15] from an excited electronic state. In all cases it is possible to scan over a resonance and thereby determine its position j4s aHd its width hkU). A schematic illustration of an absorption or emission spectrum is depicted on the left-hand side of Fig. 1 all of the more or less sharp structures at energies above threshold are resonances. Figure 2 shows an overview SEP spectrum measured for DCO [16]. It consists of... 

H.-L. Dai and R. W. Field, Eds., Molecular Dynamics and Spectroscopy by Stimulated Emission Pumping, World Scientific, Singapore, 1994. 

D. M. Neumark In Lineberger s photoelectron spectrum of the HCO" ion, resonances above the H + CO dissociation threshold were observed. Were any resonances seen in this experiment that were not seen (at higher resolution) in the stimulated emission pumping work ... 

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