Stimulated emission pumping, SEP

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.

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... 

Overtone pumping spectroscopy has the limitation that, because excitation starts in the ground vibrational state, only very specific states can be accessed, e.g., the (ui,0,0) progression in HOCl and only few other states in the vicinity of the (ui,0,0) states. Stimulated emission pumping (SEP) [147], on the other hand, involves a transition to an excited electronic state, whose equilibrium geometry may be quite different from the equilibrium in the ground state. Therefore, Pranck-Condon factors are comparatively large for a wide variety of vibrational states, not just the... 

Although work remains to be done on this problem, stimulated emission pumping (SEP) experiments strongly suggests the possibility of its occurrence. The SEP studies of highly vibrationally excited oxygen show a sharp increase in the disappearance of 02(v 26) when 02(v = 0) is the collision partner [114] and the existence of a dark channel above v = 25, where molecules prepared in a given vibrational state do not cascade down the vibrational ladder [115]. A second SEP experiment demonstrated that 02(v = 0) removes 02(v 26) at a rate 25-150 times faster than N2 [113], Subsequent studies revealed that other common atmospheric constituents such as C02 cannot compete with self-relaxation [116],... 

Figure 4.2 Three-level stimulated emission pumping (SEP) scheme, showing PUMP and DUMP transitions. The PUMP laser excites a vibrational/rotational level of an electronic state. The DUMP laser then stimulates emission from this state to an excited vibrational/rotational level of the ground electronic state (Kittrell et al., 1981).

Figure 5.12 The potential energy diagram of HFCO showing the preparation of high vibrational levels of the molecule by stimulated emission pumping (SEP). The product CO molecules are monitored by fluorescence excitation. Taken with permission from Choi et...

Figure 5.13 The stimulated emission pumping (SEP) experimental set-up. The pulsed molecular beam is located in the center of the experiment and is oriented perpendicular to the page. The pump (255 nm) and dump (375—600 nm) beams enter from the left, while the VUV fluorescence excitation (FE) laser enters from the right. Taken with permission from Choi et al. (1992).

Stimulated emission pumping (SEP) has been used to prepare resonance states for HFCO near the dissociation threshold for HF + CO formation (Choi and Moore 1991, 1992). A difference between the H2CO and HFCO potential energy surfaces, is that the HFCO —> HF + CO dissociation threshold has been estimated as 49 4 kcal/mol and is significantly smaller than the H2CO H2 + CO threshold of —85 kcal/mol. 

In the A-type OODR scheme (Fig. 5.32) 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 (SEP), may be regarded as a resonantly induced Raman-type transition. In case of monochromatic pump and probe lasers tuned to the frequencies co and co2, respectively, the resonance condition for a molecule moving with velocity v is... 

Often the super-high spectral resolution of the induced resonant Raman transitions obtained with single-mode lasers is not necessary if the levels m) are separated by more than one Doppler width. Then pulsed lasers can be used for stimulated emission pumping [600]. Many experiments on high vibrational levels in the electronic ground state of polyatomic molecules have been performed so far by SEP with pulsed lasers. Compilations may be found in [601-604]. 

U. Brinkmann, Higher sensitivity and extended frequency range via stimulated emission pumping SEP. Lambda Phys. Highlights, 1 (1990)... 

A method, femtosecond time-resolved stimulated emission pumping (SEP) fluorescence depletion (FS TR SEP ED), has been developed to study the vibrational relaxation of electronic excited states of molecules (Figures 11.9 and 11.10) [31]. 

To date, only IR photon energies in the H-stretching range have been used in IR-induced population transfer experiments, which for most systems results in excitation far above the isomerization barrier, impeding an experimental determination of the isomerization barriers directly. Zwier and coworkers presented an elegant alternative, where stimulated emission pumping (SEP) is combined with HF or PTS methods, see Fig. 2d. This combination allows one to probe the barrier to conformational isomerization [56]. The method consists of two steps a pump-dump SEP followed by a probe laser interrogation to determine the new cmifomiational distribution. In the early part of the molecular beam expansimi, SEP prepares the... 

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