Ir-pulse

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

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...

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.

An ultrashort mid-IR pulse excited a C-H stretching vibration (-3000 cm ) of neat acetonitrile at 300 K. The loss of C-H stretching energy occurred in 3 ps. Only 1% of that energy was transferred to the C N stretch (2250 cm ), where it remained for -80 ps. Most of the energy was lost from the C-H stretch by the process,... 

The use of IR pulse technique was reported for the first time around the year 2000 in order to study a catalytic reaction by transient mode [126-131], A little amount of reactant can be quickly added on the continuous flow using an injection loop and then introduce a transient perturbation to the system. Figure 4.10 illustrates the experimental system used for transient pulse reaction. It generally consists in (1) the gas flow system with mass flow controllers, (2) the six-ports valve with the injection loop, (3) the in situ IR reactor cell with self-supporting catalyst wafer, (4) the analysis section with a FTIR spectrometer for recording spectra of adsorbed species and (5) a quadruple MS for the gas analysis of reactants and products. 

Here we will focus in detail on a UV pump-IR probe spectrometer described by Emsting and co-workers the system is based on an excimer laser and a dye laser operating with a pulse repetition rate ranging from 5 to 10 Hz. Pump pulses at 308 nm excite the sample and are followed at a selected time by probe IR pulses that range from 1950 to 4300 cm Absorbance changes can be recorded with a time resolution of 1.8 ps and with an accuracy in absorbance (A) of 0.001. 

The temporal widths of the IR pulses and the time resolution of this spectrometer are tested with the use of a Ge sample that, when exposed to the pump pulses, results in transient IR absorption at 2290 cm. Modeling the risetime of this absorption gives a cross-correlation width (full width at half-maximum, fwhm) of 1.8 ps. 

D. B. Strasfeld, S.-H. Shim, and M. T. Zanni. New Advances in Mid-IR Pulse Shaping and its Application to 2D IR Spectroscopy and Ground-State Coherent Control, pages 1-28. John Wiley Sons, Inc., Hoboken, NJ (2009). 

Fig. 3. Phase Locked IR Pulses Time domain interferometry. (A) Output IR pulses from two tunable OPA-DFGs in the 4-pm frequency regime. (B) Three examples of interferograms generated by these IR pulses. (C) Linear IR absorption spectrum of acetic acid overlapped with the output of two OPAs. (D) Photon echo signal from acetic acid upon t-scan. The x-axis is the delay of the translation stage and the insert is a blow-up of a small region.

We follow the method of Ref. [4] to calculate the SFG response in the time domain. The 1R polarization is calculated by solving the Schrddinger equation in the formalism of the density matrix. It is proportional to the coherence induced by the IR pulse between v=0 and v=l. The coherence is the solution of standard coupled differential equations [6]. 

V. S. Letokhov My answer to Prof. Quack is that it is indeed difficult to predict theoretically the effect of intense femtosecond IR pulses on the IVR rate of polyatomic molecules, which is important for the transfer of vibrationally excited molecules from low-lying states to the vibrational quasi-continuum. We are developing the relevant theoretical mechanisms of IR MP E/D of polyatomics since the discovery of this effect for isotopic molecules BC13 and SF6 in 1974-1975.1 hope that it will become more realistic to study experimentally the influence of intense IR pulses on IVR due to the great progress of femtosecond laser technology. 

Short IR pulses can be generated in a number of ways. Typically, these are based on Roman scattering processes or nonlinear mixing schemes or the related optical parametric oscillator. Much more detail is given in the Stoutland reference listed. 

The pump-probe spectroscopic time-resolved study of autoionization processes in atoms and molecules uses an ultra-short (100-500 as) XUV pulses for the pump stage in conjunction with an intense (1012-1014 W/cm2), few-cycle IR pulse as probe. Traditional time-independent approaches are inadequate to interpret these kind of experiments. This is so because, on the... 

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