Raman pulsed

The fast-forward protocol can be used to assist an incomplete STIRAP population transfer via control of the time dependences of the phases of the fields. As a first example, we reconsider transfer of particles between sites of a linear lattice. In this example, the hopping rates, which connect nearest neighbor sites, are the analogs of the Stokes and Raman pulses in a conventional STIRAP. We now allow the hopping rates to change with time. The derivation of the fast-forward driving potential proceeds in the same manner as Section 3.3.5. The Schrodinger equation for an accelerated state T pp is... 

Low frequency spectra of liquids are notably deficient of any structure, and it has long been hoped that a technique would be discovered that provides the same type of line narrowing enjoyed in echo-based electronic and NMR spectroscopy. Tanimura and Mukamel observed that such a technique was possible, and proposed a two-time interval, fifth-order Raman pulse sequence capable of distinguishing, for example, inhomogeneous and homogeneous contributions to the lineshape.[4] The pulse sequence, shown in Fig. 1, is simply an extension of conventional time-domain third-order Raman-based methods. At the... 

Fig. 1. Two showing the fifth-order Raman pulse sequence. (A) Definition of fields, (B) Energy level diagram showing one possible Liouville space pathway.

The parameters used in Fig. 1 for a BECBose-Einstein condensate of 87Rb may correspond to impurity atoms of the same isotope but in a different hy-perfine state, obtained by a short Raman pulse. This method of impurity atom admixing conforms to the initial conditions of Eq. (2). However, to reach appreciable non-exponentiality by this method, one has to enhance interspecies scattering either by means of interspecies Feshbach resonance or via laser-... 

FIGURE 16.3 The scheme for Raman spectroscopy of Sr2 ground-state vibrational spacings. A two-color photoassociation pulse prepares molecules in the v = Umax — 2 vibrational state (labeled in the figure as u = —3). Subsequently, a Raman pulse couples the v = —3 and v = 21 states via the u 40 level of the excited 0 state. (From Zelevinsky, T. et al., Phys. Rev. Lett., 100, 043201, 2008 arXiv 0708.1806, 2007. With permission.)... 

Single-qubit rotations can be accomplished with optical or microwave fields. The initial states of two individual sites a and b can be prepared in a superposition state, for example, using jt/2 Raman pulses. 

The pioneering use of wavepackets for describing absorption, photodissociation and resonance Raman spectra is due to Heller [12, 13,14,15 and 16]- The application to pulsed excitation, coherent control and nonlinear spectroscopy was initiated by Taimor and Rice ([17] and references therein). 

Time-resolved spectroscopy has become an important field from x-rays to the far-IR. Both IR and Raman spectroscopies have been adapted to time-resolved studies. There have been a large number of studies using time-resolved Raman [39], time-resolved resonance Raman [7] and higher order two-dimensional Raman spectroscopy (which can provide coupling infonuation analogous to two-dimensional NMR studies) [40]. Time-resolved IR has probed neutrals and ions in solution [41, 42], gas phase kmetics [42] and vibrational dynamics of molecules chemisorbed and physisorbed to surfaces [44]- Since vibrational frequencies are very sensitive to the chemical enviromnent, pump-probe studies with IR probe pulses allow stmctiiral changes to... 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

With the advent of short pulsed lasers, investigators were able to perfonn time resolved coherent Raman scattering. In contrast to using femtosecond pulses whose spectral widtii provides the two colours needed to produce Raman coherences, discussed above, here we consider pulses having two distinct centre frequencies whose difference drives the coherence. Since the 1970s, picosecond lasers have been employed for this purpose [113. 114], and since the late 1980s femtosecond pulses have also been used [115]. Flere we shall briefly focus on the two-colour femtosecond pulsed experiments since they and the picosecond experiments are very similar in concept. 

As already mentioned, electronically resonant, two-pulse impulsive Raman scattering (RISRS) has recently been perfonned on a number of dyes [124]. The main difference between resonant and nom-esonant ISRS is that the beats occur in the absorption of tlie probe rather than the spectral redistribution of the probe pulse energy [124]. These beats are out of phase with respect to the beats that occur in nonresonant ISRS (cosinelike rather tlian sinelike). RISRS has also been shown to have the phase of oscillation depend on the detuning from electronic resonance and it has been shown to be sensitive to the vibrational dynamics in both the ground and excited electronic states [122. 124]. 

Yan Y X, Gamble E B and Nelson K A 1985 Impulsive stimulated Raman scattering general importance in femtosecond laser pulse interactions with matter, and spectroscopic applications J. Chem. Phys. 83 5391-9... 

Siebert D R, West G A and Barrett J J 1980 Gaseous trace analysis using pulsed photoacoustic Raman spectroscopy Appl. Opt. 19 53-60... 

Another important breaktlirough occurred with the 1974 development by Laubereau et al [24] of tunable ultrafast IR pulse generation. IR excitation is more selective and reliable than SRS, and IR can be used in pump-probe experiments or combined with anti-Stokes Raman probing (IR-Raman method) [16] Ultrashort IR pulses have been used to study simple liquids and solids, complex liquids, glasses, polymers and even biological systems. 

Schematic diagrams of modem experimental apparatus used for IR pump-probe by Payer and co-workers [50] and for IR-Raman experiments by Dlott and co-workers [39] are shown in figure C3.5.3. Ultrafast mid-IR pulse generation by optical parametric amplification (OPA) [71] will not discussed here. Single-colour IR pump-probe or vibrational echo experiments have been perfonned with OP As or free-electron lasers. Free-electron lasers use...

Por IR-Raman experiments, a mid-IR pump pulse from an OPA and a visible Raman probe pulse are used. The Raman probe is generated either by frequency doubling a solid-state laser which pumps the OPA [16], or by a two-colour OPA [39]. Transient anti-Stokes emission is detected with a monocliromator and photomultiplier [39], or a spectrograph and optical multichannel analyser [40]. 

With broad-band pulses, pumping and probing processes become more complicated. With a broad-bandwidth pulse it is easy to drive fundamental and overtone transitions simultaneously, generating a complicated population distribution which depends on details of pulse stmcture [75], Broad-band probe pulses may be unable to distinguish between fundamental and overtone transitions. For example in IR-Raman experiments with broad-band probe pulses, excitation of the first overtone of a transition appears as a fundamental excitation with twice the intensity, and excitation of a combination band Q -t or appears as excitation of the two fundamentals 1761. 

Figure C3.5.11. IR-Raman measurements of vibrational energy flow tlirough acetonitrile in a neat liquid at 300 K, adapted from [41], An ultrashort mid-IR pulse pumps the C-H stretch, which decays in 3 ps. Only 1% of the energy is transferred to the C N stretch, which has an 80 ps lifetime. Most of the energy is transferred to the C-H bend plus about four quanta of C-C=N bend. The daughter C-H bend vibration relaxes by exciting the C-C stretch. The build-up of energy in the C-C=N bend mirrors the build-up of energy in the bath, which continues for about 250 ps after C-H stretch pumping.

---