Stimulated emission pumping ground electronic state

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.

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... 

Population inversion cannot be achieved in a two-level system, a material with two electronic states. At best, a nearly equal population of the two states is reached, resulting in optical transparency, when absorption by the ground state is balanced by stimulated emission from the excited state. An indirect method of populating the emitting excited state must be used. In a three-level laser (Figure 3.6, left), irradiation of the laser medium pumps an upper level 2, which is rapidly depleted by a nonradiative... 

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).

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]. 

A very interesting optical cooling technique starts with the selective excitation of a collision pair of cold atoms into a bound level in an upper electronic state (Fig. 9.17). While this excitation occurs at the outer turning point of the upper-state potential, a second laser dumps the excited molecule down into a low vibrational level of the electronic ground state by stimulated emission pumping (photo-induced association). In favorable cases the level u = 0 can be reached. If the colliding atoms... 

K. Yamanouchi, H. Yamada, S. Tsuciya, Vibrational levels structure of highly excited SO2 in the electronic ground state as studied by stimulated emission pumping spectroscopy. J. Chem. Phys. 88, 4664 (1988)... 

It has been shown that in the limit of ultrashort laser pulses the stimulated-emission pump-probe signal is proportional to the population probability of the initially excited diabatic state [Tf)) Eq. (59) and Refs. 7, 99 and 141. As has been emphasized in Chapter 9, the electronic population probability P2 t) represents a key quantity in the discussion of internal-conversion processes, as it directly reflects the non-Born-Oppenheimer dynamics (in the absence of vibronic coupling, P2 t) = const ). It is therefore interesting to investigate to what extent this intramolecular quantity can be measured in a realistic pump-probe experiment with finite laser pulses. It is clear from Eq. (33) that the detection of P2(t) is facilitated if a probe pulse is employed that stimulates a major part of the excited-state vibrational levels into the electronic ground state, that is, the probe laser should be tuned to the maximum of the emission band. Figure 4(a) compares the diabatic population probability P2(t) with a cut of the stimulated-emission spectrum for uj2 3.4 eV, i.e. at the center of the red-shifted emission band. Apart from the first 20 fs, where the probe laser is not resonant with the emission [cf Fig. 2(b)], the pump-probe signal is seen to capture the overall time evolution of electronic population probability. Pump-probe experiments thus have the potential to directly monitor electronic populations and thus non-Born-Oppenheimer dynamics in real time. ... 

In a typical pump-probe experiment, a sample is excited with a pulse with frequency < i and wavevector ki, and is probed by a second pulse with frequency C02 and wavevector 2- The optical path of (Mie of the pulses is varied to change the delay between the two pulses. The measured signal is the difference between the intensities of the transmitted probe pulses in the presence and absence of the excitation pulses, and usually is averaged over many pulses (Fig. 1.9). In a system with only two electronic states, the difference can reflect either stimulated emission from the excited state or bleaching of the absorption band of the ground state. The probe frequency often is selected by dispersing a spectrally broad probe beam after... 

In the preceding sections we discussed cases in which the molecule is first electronically excited to an upper-state PES before it dissociates, in the second step, on this PES. Dissociation can, of course, also take place directly on the ground-state PES, without detour via an excited state. This process is known as unimolecular dissociation and plays an important role in combustion processes or atmospherical chemistry (see Rates of Chemical Reactions and Unimolecular Reaction Dynamics). Experimentally, unimolecular dissociations can be directly measured either by overtone excitation (OH stretching vibration, for example) or by. stimulated emission pumping from an excited electronic state. On the theoretical side, all... 

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