Deexcitation pulse

We now pose the question Given the most general shape of an exciting and stimulating (deexciting) pulse, which shape is optimal in producing a desired chemical product A little reflection shows that this is a problem in the calculus... 

We will solve separately for the excitation pulse, a(t,), and the deexcitation pulse, b(t2), beginning with the latter. Beginning with Eq. (5.11) we have... 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

A little more complicated system is the de-excitation of He(2 P) by Ne, where the deexcitation is dominated by the excitation transfer and only a minor contribution from the Penning ionization is involved. The experimental cross section obtained by the pulse radiolysis method, together with the numerical calculation for the coupled-channel radial Schrodinger equation, has clearly provided the major contribution of the following excitation transfer processes to the absolute de-excitation cross sections [151] (Fig. 15) ... 

As a brief conclusion of this section, the cross-section measurements for the deexcitation of excited rare gas atoms have been best performed using the pulse radiolysis method. The pulse radiolysis method has provided not only the most reliable cross sections... 

In the discussion so far, instantaneous excitation or deexcitation by a 8 function pulse has been assumed to transfer wave packets from one electronic state to another state. For realistic pulses, the wave packets may be obtained by numerically integrating Eqs. (25) and (30). 

Optical excitation of metals with intense femtosecond laser pulses can create extreme non-equilibrium conditions in the solid where the electronic system reaches several thousand degrees Kelvin on a sub-picosecond timescale, while the lattice (phonon) bath, stays fairly cold. As illustrated in Figure 3.22, photoexcited hot electrons may transiently attach to unoccupied adsorbate levels and this change in the electronic structure may induce vibrational motions of the adsorbate-substrate bond. For high excitation densities with femtosecond pulses, multiple excitation/deexcitation cycles can occur and may eventually lead to desorption of adsorbate molecules or reactions with co-adsorbed species. After 1-2 ps, the hot electron... 

One of the main problems met in Laser Induced Fluorescence measurements is the excited population dependence on the quenching due to collisional deexcitation. The saturation mode proposed to avoid this dependence is very difficult to achieve U ) (2 ) particularly with molecular species and moreover the very strong laser pulses required may cancel the non-perturbing characteristic of the method. Therefore precise knowledge of the quenching is necessary in some experimental circumstances. 

If J" —> J excitation is accompanied or followed by deexcitation J —> J" in a stimulated emission process (SEP), then the population efficiency of the level can be increased considerably. It is now known [248, 347] that the process might be made more effective by applying the A-configuration scheme in which the first-step (J" — J ) excitation pulse is applied after the second-step (J — J") pulse which, at first glance, seems surprising. This process is called stimulated Raman scattering by delayed pulses (STIRAP). The population transfer here takes place coherently and includes coordination of the Rabi nutation phase in both transitions. 

The first pulse is used to excite a pair of states in an electronic state supporting j bound states, and the second pulse is used to dissociate the system by deexciting it I back to the ground state, above the dissociation threshold. 

Fig. 4A)-C) show time spectra when two successive laser pulses are applied. In Fig. 4A), the two pulses have different frequencies, and therefore the pulse height is determined by the population of the HF levels at times t and 2 when the pulses are applied. In Fig. 4B) two pulses of the same frequency are applied. In this case the laser peak at 2 is much smaller than the one at 1, its height being determined by the deexcitation efficiency of the first pulse, plus feeding from upper states during the period between the two pulses.

We consider a system for which there can be a reaction on the ground-electronic-state potential energy surface, and ask how that reaction can be mediated by excitation to, evolution on, and stimulated deexcitation from, an excited electronic state. The excitation and stimulation pulse shapes, durations, and separations required to achieve selectivity of product formation depend on the properties of the excited-state potential energy surface. In the relevant time domain, which is defined by the shape of the excited-state potential energy surface, we shall show that it is possible to take advantage of the localization in phase space of the time-dependent quantum mechanical amplitude and thereby carry our selective chemistry. 

The second scheme of coherent control, proposed by D.J. Tanner and A. Rice, utilizes the time difference between two femtosecond laser pulses interacting with a molecule on two different transitions sharing a common level, which was illustrated in Sect. 6.4.4 by the example of the Na2 molecule. Here the phase of the wave packet produced by the first pulse in the excited state develops in time, and the controlled time lag between the first and the second pulse selects a favorable phase for further excitation or deexcitation of the molecule by the second pulse. 

One wants to achieve population inversion, meaning that level 2 has a higher occupation than level 1. When the electrons start to deexcite and reoccupy the ground state, the intensity starts to increase in the frequency range of the emission. The deexcitation is now a stimulated emission and its intensity increases manifold. The laser is fired by itself and a light pulse is emitted. The emitted light penetrates the semitransparent mirror. 

For pulses of shorter durations, we may regard the results as the formation and time evolution of vibrational wavepackets on (the not dressed) electronic states and exploit the timing of electronic excitation/deexcitation [434], directing the wavepacket s destination [324] or avoiding certain potential energy surface features [281]. Alternately, several wavepackets may be formed and the interference among them exploited for control [306]. 

The pump pulse causes 74% excitation into the A state (between t = —8 and 4 fs). Afterwards the probe pulse train causes transfer of population between the A and B states, and between B and C (and X) states. Transfer of population occurs synchronous with the pulses in the pulse train, causing the step-like appearance seen in Fig. 5.35(a). The pulse train causes ionization simultaneously with population transfer among the neutral states. Time evolution of the ionized population is also seen to be step-like, with change synchronous with the pulses. Thus the electronic excitation and deexcitation seems to be a Rabi oscillation whose timing of transition is well controlled. On the other hand, the relevant nuclear vibrational motion is autonomously evolved in time during the refractory period. 

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