Raman heterodyne spectroscopy

To summarize, the EOM-PMA considerably facilitates the computation of various optical signals and 2D spectra. With shght alterations, the EOM-PMA can also be applied to compute nonlinear responses in the infrared (IR). The three-pulse EOM-PMA can be extended to calculate the A-pulse-induced nonhnear polarization [51], which opens the way for the interpretation of fifth-order spectroscopies, such as heterodyned 3D IR [52], transient 2D IR [53, 54], polarizability response spectroscopy [55], resonant-pump third-order Raman-probe spectroscopy [56], femtosecond stimulated Raman scattering [57], four-six-wave-mixing interference spectroscopy [58], or (higher than fifth order) multiple quantum coherence spectroscopy [59]. 

Eesley, G. L., Levenson, M. D., and Tolies, W. M. 1978. Optically heterodyned coherent Raman spectroscopy. IEEE J. Quant. Electron. 14 45-49. 

Before melting and for some time after only the band at 625 cm of the AA [C4CiIm]+ cation was observed in the 600-630 cm i region. Gradually 603 cm i band due to the GA conformer became stronger. After about 10 min the AA/GA intensity ratio became constant. The interpretation [50] is that the rotational isomers do not interconvert momentarily at the molecular level. Most probably it involves a conversion of a larger local structure as a whole. The existence of such local structures of different rotamers has been found by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [82], Coherent anti-Stokes Raman scattering (CARS) [83],... 

The following section contains a more detailed treatment of the theory behind the nonresonant spectroscopy of liquids. This will be followed by a description of the experimental implementation and data analysis techniques for a common OKE scheme, optical-heterodyne-detected Raman-induced Kerr-effect spectroscopy (22). We will then discuss the application of this technique to the study of the temperature-dependent dynamics of simple liquids composed of symmetric-top molecules. 

Cong P, Deuel HP, Simon JD. Structure and dynamics of molecular liquids investigated by optical-heterodyne detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). Chem Phys Eett 1995 240 72-78. 

Femtosecond optical heterodyn-detected optical Kerr effect spectroscopy and low-frequency Raman spectroscopy were used to study the molecular dynamics of selenophene <1998JCP10948>. Femtosecond Kerr effect spectroscopy was also used to examine the third-order polarizabilities of furan, thiophene, and selenophene, which was found to increase from furan to thiophene to selenophene <1996CPL(263)215>. 

OHD-RIKES(Optical Heterodyne Detected Raman Induced Kerr Effect Spectroscopy)... 

Fig. 2 Experimental arrangement for time-resolved FSRS (femtosecond stimulated raman spectroscopy). The femtosecond actinic pump pulse excites the sample electronically. After a delay the femtosecond probe pulse and picosecond Raman pump pulse arrive together to interrogate the instantaneous molecular structure. The self-heterodyned signal is emitted in the probe direction, dispersed, and detected by a kHz readout CCD. Data collection is best performed by division of subsequent Raman pump-on by Raman pump-off laser shots (lower trace), however this has been performed by other groups as a subtraction of subsequent pulses (upper trace). Reproduced from ref 2 with permission from the PCCP Owner Societies (2012).

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

A new technique to measure low-frequency spectra is optical-heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES). A recent publication by Chang and Cast-ner contains references to previous work within this field [18]. OHD-RIKES is based on a four-wave mixing of femtosecond laser pulses. Spectra obtained by OHD-RIKES reflect the anisotropic part of the Raman polarizability. Thus, the information obtained by OHD-RIKES is very similar to that obtained by low-frequency Raman scattering in an scattering configuration. From a theoretical point of view, the spectral representation obtained from OHD-RIKES measurements corresponds to the I v) representation given in Eq. (3). In Fig. 4 is shown an OHD-RIKES spectrum of liquid A-methylformamide (NMF). In Fig. 5 are shown low-frequency Raman spectra of liquid NMF together with the R(i>),... 

K. A., Rogers, J.A. (1998). Optical heterodyne detection of laser-induced gratings. Opt. Lett. 23 1319-1321 Fecko,C.J.,Eaves, J.D.,Tokmakoff, A. (2002). Isotropic and anisotropic Raman scattering from molecular liquids measured by spatially masked optical Kerr effect spectroscopy. J. Chem. Phys. 117 1139-1154. 

---