Rotational coherence spectroscopy

Different schemes and recent results for high-resolution rotational coherence spectroscopy with picosecond and femtosecond laser pulses... 

Over the last years we have explored several advanced techniques for high-resolution rotational coherence spectroscopy (RCS [1]) in order to study the structures of molecules and clusters in the gas phase [2]. We have provided spectroscopic examples demonstrating (i) mass-selectivity (Fig. 1, [3]), (ii) that the rotational constants of the ground and electronic excited states can be obtained independently with high precision (lO MO"5, [4]), (iii) that the transition dipole moment alignment, (iv) centrifugal distortion constants, and (v) information on the polarizability tensor can be obtained (Fig.l, [5]). Here we review results pertaining to points (i), (ii), (iv) and (v) [2,3,5],... 

Fig, 1. Pump-probe photoionization (PPI) and time-resolved degenrate four-wave mixing (TRDFWM) schemes for rotational coherence spectroscopy (RCS). 

DilTcrcm schemes and recent results for high-resolution rotational coherence spectroscopy with picosecond and femtosecond laser pulses C. Kiehn, V.-V Matylitsky. A. Weiehert, M.-F. Gclin. W. Jar/eba and B Brutschy 73... 

In extending the studies of vibrational coherence to rotational coherence in isolated molecules, we formulated the concept of rotational recurrences (echoes ), which led to rotational coherence spectroscopy. A polarized picosecond (and later femtosecond) pulse was used to orient a molecular ensemble (Fig. 7). The molecules then rotate freely with different speeds... 

Figure 7. (a) Concept of time-dependent alignment as a method for structural determination. Top Initial alignment at t = 0, dephasing, and recurrence of alignment at later times. Bottom Classical motion of a rigid prolate symmetric top. (b) Structures of stilbene and tryptamine-water complex from rotational coherence spectroscopy transients are shown, [see ref. 13]. 

A. H. Zewail To provide a partial answer to the question of Prof. Hamaguchi, the structure of the 1 1 stilbene-hexane species was determined with the help of rotational coherence spectroscopy. For higher clusters we used atom-atom model potentials and deduced structures. 

The structure of the 1 1 BA-H2O complex in its ground state has been determined by rotational coherence spectroscopy (RCS) using the fluorescence depletion scheme [74, 75], The empirical minimum-energy calculation supplemented the insufficient information obtained from the RCS measurement [74], In the structure giving the best fit to the RCS signal, the water molecule sits on the terminal aromatic ring of the one anthracene moiety. Thus the two anthracene moieties are stabilized by the water molecule in a different manner, and such asymmetry in the stabilization, i.e., symmetry breaking, facilitates the electron jump from one anthracene to the other [76]. 

TMA acetate IR PhOH rotational coherence spectroscopy -F B3LYP/6-31G(d) 634, 635 636... 

Rotational reorientation of frans-stilbene in alkane solution at room temperature occurs in the 10 to 30-ps time domain [347]. Rare-gas complexes with trons-stilbene were studied by purely rotational coherence spectroscopy [51,364]. Moreover, the decay kinetics of excited trans-stil-bene-cyclodextrin complexes were examined [366], It is worth mentioning that great progress has also been made in high-resolution spectroscopy [52, 369-372], Resonance coherent Raman spectroscopy showed a large enhancement of the electronic hyperpolarizability of t with respect to ground state trons-stilbene [374]. Vibrational motions were observed with ps transient Raman spectroscopy [375]. 

This suggests that a quantum beat spectrum of an AT-level system will, because it contains redundant information, be more complicated than the corresponding frequency domain spectrum. However, when the level spacings are approximately integer multiples of a common factor, such as 2B for upper-state A2F(J) = B (iJ + 2) rotational combination differences, then each upper state (J + 1, J — 1) pair of rotational levels coherently excited from all thermally populated lower-state J" levels contributes to a grand rephasing at tn = n [-gj ] (n = 1,2,...). This is Rotational Coherence Spectroscopy (RCS) (Felker and Zewail, 1987 and 1995 Felker, 1992). It provides upper state rotar tional constants without the need for a rotational analysis. 

In particular, the laboratory frame orientation of the transition moment for spontaneous fluorescence evolves in time. The intensities of z— and (x,y) — polarized fluorescence are modulated 7t/2 out of phase, but the intensity of the total x + y + z polarized fluorescence is not modulated. This is the physical basis for polarization quantum beats (Aleksandrov, 1964 Dodd, et al., 1964) and Rotational Coherence Spectroscopy (Felker and Zewail, 1995). 

Connell, L.L., T. Corcoran, P. Joireman, and P. Felker. 1990. Conformational analysis of jet-cooled tryptophan analogs by rotational coherence spectroscopy. Chem. Phys. Lett. 166 510-516. 

Purely Rotational Coherence Effect and Time-Resolved Sub-Doppler Spectroscopy of Large Molecules, P. M. Felker, J. S. Baskin, and A. H. Zewail, J. Chem. Phys. 86,2460 and 2483... 

Baskin, J.S., Felker, P.M. and Zewail, A.H. (1987). Purely rotational coherence effect and time-resolved sub-Doppler spectroscopy of large molecules. II. Experimental, J. Chem. Phys., 86, 2483-2499. 

PURELY ROTATIONAL COHERENCE AND SUB-DOPPLER SPECTROSCOPY. Guided by the theoretical decay simulations of Fig. 46, the first unambiguous observation of thermally averaged rotational coherence effects was made for excitation and detection of the S, - S00° band of jet-cooled t-stilbene.47 Observed fluorescence decays are shown in Fig. 47 theory and experiment match very well. The recurrences associated with rotational coherence effects in fluorescence have been observed for a number of other species as well. Among these species are t-stilbene-, 2, t-stilbene-argon complexes,48 and t-stilbene-he-lium complexes.71 The recurrences allow the determination of the excited-state rotational constants to a high degree of accuracy. [For example, for t-stilbene we find j(B + C) to be 0.00854 + 0.00004 cm-1.] The indications are that with currently available temporal resolution, rotational coherence effects should be observable in a multitude of species and should allow the accurate determination of such species excited-state rotational constants. 

Vestin, R, Sedarsky, D., Collin, R., Alden, M., Linne, M., and Bengtsson, R. "Rotational Coherent Anti-Stokes Raman Spectroscopy (CARS) Applied to Thermometry in High Rressure Flames." Combustion and Flame 154 (2008) 143. 

F. Moya, S.A.J. Druet, J.P. Taran Rotation-vibaration spectroscopy of gases by coherent anti-Stokes Raman scattering Application to concentration and temperature measurements, in Laser Spectroscopy, ed. by S. Haroche, J.C. Pebay Peyroula, T.W. H nsch, S.E. Harris (Springer, Berlin, Heidelberg 1975)... 

Much of the previous section dealt with two-level systems. Real molecules, however, are not two-level systems for many purposes there are only two electronic states that participate, but each of these electronic states has many states corresponding to different quantum levels for vibration and rotation. A coherent femtosecond pulse has a bandwidth which may span many vibrational levels when the pulse impinges on the molecule it excites a coherent superposition of all tliese vibrational states—a vibrational wavepacket. In this section we deal with excitation by one or two femtosecond optical pulses, as well as continuous wave excitation in section A 1.6.4 we will use the concepts developed here to understand nonlinear molecular electronic spectroscopy. 

The best resolution of Q-branch rotational structure in a N2-Ar mixture was achieved by means of coherent anti-Stokes/Stokes Raman spectroscopy (CARS/CSRS) at very low pressures and temperatures (Fig. 0.4). A few components of such spectra obtained in [227] are shown in Fig. 5.9. A composition of well-resolved Lorentzian lines was compared in [227] with theoretical description of the spectrum based on the secular simplification. The line widths (5.55) are presented as... 

Temkin S. I., Thuet J. M., Bonamy L., Bonamy J., Robert D. Angular momentum and rotational energy relaxation in N2-N2 collisions calculated from coherent and stimulated Raman spectroscopy data,... 

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