Stimulated Raman Pumping

Fig. 13. Schematic view of the irradiated STIRAP process (Stimulated Raman pumping) (a) The chirped pump pulse excites the system the electronic ground state (b) the system is stimulated back down to the electronic ground state, but a Stokes-shifted higher vibrational level, (c) The resulting wave packet is probed by the delayed probe pulse, which ionizes the particle from the oscillating ground state.

Carbon. - Stimulated Raman pumping-anion photoelectron spectroscopy showed that the l<-0 vibrational transition of C2 is at 1757.8 + 0.1 cm h For M2C2, where M = Rb or Cs, the [C2] anion vCC mode is at 1807 cm (Rb, orthorhombic), 1805 cm (Rb, hexagonal) or 1796 cm (Cs). These values are consistent with trends from the lighter alkali metal analogues. ... 

An investigation of the overtone region 21 1 and 2v of CH4 by spontaneous Raman spectroscopy [231] did not allow a precise dermination of the position and rotational structure of these states, which are part of a tetradecad system. Through population of the excited state by stimulated Raman pumping and subsequent quasi-CW stimulated Raman spectroscopy, the difference band 2v — could be observed and analyzed recently [232]. Figure 25 shows the transitions from the ground state to P2 + P4 (top) which appear in this region, the additional spectrum of the difference band 2p — Pi (middle), and the theoretical spectrum calculated with the polyads model (bottom). 

Figure 3. Partial representation of frequencies available from the Nd. YAG fundamental (1) harmonics (2-5), YAG-pumped dye laser fundamental (Dye O), and stimulated Raman H2 and D2 shifts thereof. The Dye regions are approximations to the average envelopes of four dyes Coumarin 500, and Rhodamines 590, 610,...

Figure 4. The sample cell arrangement in the DCSHG experiment, where the sample solution was inserted between two glass slips (lop), and the optical design for the DCSHG dispersion experiment, where the compressed H gas medium was pumped by a tunable pulsed dye laser source for Stokes generation by stimulated Raman scattering (bottom). (E° is the static electric field.) Key beam guiding prisms P, Stokes...

The first observation of the stimulated Raman effect was reported by Woodbury and Ng 215) j e effect was then thoroughly studied by several authors 216-218) and its theoretical background developed 219.220) (see also the review articles by Zubov et a/.22D). The stimulated Raman effect can be described as a parametric process where the coupling between a light wave at the Stokes frequency (Os and an optical phonon (vibrational wave) at cOy is produced by a pump field at col = (Oj + ojy. 

Coherent excitation of quantum systems by external fields is a versatile and powerful tool for application in quantum control. In particular, adiabatic evolution has been widely used to produce population transfer between discrete quantum states. Eor two states the control is by means of a varying detuning (a chirp), while for three states the change is induced, for example, by a pair of pulses, offset in time, that implement stimulated Raman adiabatic passage (STIRAP) [1-3]. STIRAP produces complete population transfer between the two end states 11) and 3) of a chain linked by two fields. In the adiabatic limit, the process places no temporary population in the middle state 2), even though the two driving fields - pump and Stokes-may be on exact resonance with their respective transitions, 1) 2)and... 

STIRAP Transfer of population by means of Stimulated Raman Adiabatic Passage, using a pump and Stokes laser. Population in a three-level system is completely transferred without populating the intermediate state if the Stokes laser precedes the pump laser in a counterintuitive order. 

Population transfer in a three-level system can be achieved by using one laser (known as the pump laser, which may be either continuous wave or pulsed) to connect the ground and intermediate levels, and a second laser (the Stokes laser ) to connect the intermediate and final levels. This method, known as stimulated Raman adiabatic passage or STIRAP, is illustrated in Fig. 22. In this example, the three levels have a A-type configuration, where... 

Pulsed laser-Raman spectroscopy is an attractive candidate for chemical diagnostics of reactions of explosives which take place on a sub-microsecond time scale. Inverse Raman (IRS) or stimulated Raman loss (.1, ) and Raman Induced Kerr Effect (2) Spectroscopies (RIKES) are particularly attractive for singlepulse work on such reactions in condensed phases for the following reasons (1) simplicity of operation, only beam overlap is required (2) no non-resonant interference with the spontaneous spectrum (3) for IRS and some variations of RIKES, the intensity is linear in concentration, pump power, and cross-secti on. 

Further, as discussed in Section 3.1, the inability to control the product ratio by shaping the pulse can be overcome by photodissociating not just one EX) bound . state but a superposition of several bound states )) (as was done, e.g., with bichro-, matic control). Such a superposition state can be created separately by an initial preparation pulse, as in the case of pump-dump control scenario Sections 3.5 and. 4.1). Alternatively, the superposition state can be created by the photolysis pulse itself (by, e.g., a stimulated Raman process), provided that the bandwidth of the -pulse is comparable to the energy spacings between the Ef) levels. r, ... 

Although the peak power of the pump laser must be high, the power of the probe laser should be kept low to avoid nonlinear effects (multiphoton absorption, stimulated Raman scattering see Section 3.9.2) and dielectric breakdown (ionization of molecules) that damage the sample. Thus, signal averaging of many pulses (high repetition rate) is made to obtain acceptable S/N ratios. Multichannel detectors such as an intensified silicon photoiode... 

Figure 3-43 Schematic representation of the photoacoustic Raman scattering (PARS) process, (a) A simple energy level diagram illustrating the Raman interaction that occurs in the PARS process, (b) Basic elements of the PARS experimental arrangement. The pump beam is attenuated and the Stokes beam is amplified by the stimulated Raman process that takes place where the beams overlap in the gas sample cell. For each Stokes photon created by the Raman process, one molecule is transferred from the lower state to the upper state of the transition. Collisional relaxation of these excited molecules produces a pressure change that is detected by a microphone. (Reproduced with permission from Ref. 107.)...

A major breakthrough in the measurement of VER occurred in 1972. Laubereau et al. (32) used picosecond laser pulses to pump molecular vibrations via stimulated Raman scattering (SRS) and time-delayed incoherent anti-Stokes probing to study VER of C-H groups in ethanol and methanol ( " -3000 cm-1). Alfano and Shapiro (33) used the same technique to monitor both the decay of the initially excited (parent) C-H stretch excitation and the appearance and subsequent decay of a daughter vibration,... 

A nanosecond temperature jump is induced by an illumination of a sample with the laser heat pump. For instance, in the work of Yamamoto et al. (2000) the 1.56-pm heat pulse 9 ns width at 10 Hz was obtained through the two-step stimulating Raman scattering in D2 gas. 

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