Broad-band excitation from pulse

FIGURE 43. Assignment of the two lines in the 29Si NMR spectmm of methyl 3o, 12a-bis(trimethyl-siloxy)-5/i-cholenate, 37, by selective INEPT. Top trace 29Si INEPT spectmm two middle traces selective INEPT spectra measured with selective excitation of H lines indicated by arrows in the bottom trace with partially assigned ll NMR spectmm (25 mg of the sample in 0.7 ml of CDCI3, H frequency 200 MHz, 29Si frequency 39.7 MHz, 5 mm broad-band probe, selective pulse by DANTE train, r = 70 ms, A = 149 ms). Reproduced with permission of Collection of Czechoslovak Chemical Communications from Reference 304... 

Fluorescence from singlet excited states of [Re(X)(CO)3(bpy)] (X = Cl, Br, I) and [Re(Etpy)(CO)3(bpy)]+ appears concomitantly with 400 mn, 80 fs pulse excitation as a very broad band peaking at -530 nm (in Chap. 1 of this book). The large instantaneous fluorescence Stokes shift and broadness indicate a femtosecond energy dissipation and ultrafast population of many vibrational modes. TD-DFT calculations [10]... 

The photochemical ligand substitution reaction of la was investigated by ultrafast TR-IR spectroscopy (Fig. 16) 51). An acetonitrile solution of la was irradiated by a 266-nm laser pulse ( 150 fs pulse width). A broad IR absorption band which was attributed to the reaction products in higher vibrational excited states was produced within 1 ps after the laser flash. The broad band sharpened and a vqo peak at 1828 cm of the reaction product was observed in the 50- to 100-ps duration. This time scale is much shorter than the decay of the lowest MLCT excited state (right-hand side of Fig. 16). The TR-IR results indicate that this photochemical reaction proceeds from higher vibrational states or high-energy electronic excited states instead of the lower vibrational excited states of MLCT and thermal accessible states from MLCT such as the LF state. 

Porphyrin cation radical species as intemiediates in ET can be directly detected by laser-flash photolysis or ESR spectroscopy. In particular, transient absorption spectra of porphyrin-acceptor assembly systems excited by laser pulses often show the characteristic broad band assigned to porphyrin cation radical species at 500-800nm," so it is possible to determine the forward and/or back ET rate constants from the formation and decay profiles of the intermediate absorption. In noncovalent ET model systems, sufficient affinity between the porphyrin and the acceptor is required to obtain an appropriate inten,sity of transient absorption derived from porphyrin cation radical species. However, it is difficult to monitor the intermediate when charge recombination processes occur faster than the charge-separation process. 

Nevertheless, it is possible to give a formal description of a statistical-limit molecule in the same terms as previously used in the strong-coupling case. It is well known that the emission spectrum of large molecules (studied up to now only in condensed phases) is composed of narrow bands (considered as the resonance Raman scattering) and broad-band fluorescence. The relative intensity of the first component is enhanced in presence of fluorescence quenchers (Friedman and Hochstrasser, 1975), or in laser intracavity experiments (Bobovich and Bortkevich, 1977). The first component may be related to the emission from nonstationary s> states with redistribution time shorter than the exciting-pulse duration. The second component would be due to the rapid vibrational redistribution. In the limiting case of nonfluorescent molecules only the resonance Raman spectrum persists. The nonradiative deactivation of the excited state would be more rapid here than the vibrational redistribution. 

Using high intensity (especially pulsed) lasers it is possible to dissociate the methanol dimer by two-photon excitation. Again a two-peak dissociation spectrum about twice as broad as the one that is found for pure excitation is observed without essential shifts. Embarrassing is the fact that also beams seeded in He show this two-peak spectrum (only slightly broadened) [11], in contrast to the observations discussed in section 2, where optothermal detection yielded very broad banded dissociation spectra (FWHM of 40 cm ) after absorption of a single photon. To explain this discrepancy we have to keep in mind that the detection efficiency E is for the dissociated dimers -caused by one or two-photon transitions as it may be - is different from that of excited dimers Eex- for optothermal detectors (see Table 1). 

The excitation pulse used in Fig. 11.17 was assumed to include a broad band of energies relative to the vibrational energy spacing ho. In this rather special situation, X u,t) has a Gaussian shape that remains constant indefinitely as the wavepacket oscillates. Such wavepackets provide a useful description of the radiation emitted from a continuous-wave laser. In this picture, the spatial oscillations of the wavepacket resemble the oscillating electric field associated with a continuous stream of photons with constant energy [118]. 

The best results are to be expected from stored waveform inverse Fourier transform (SWIFT) excitation [194]. First, the ideal excitation waveform is tailored to the needs of the intended experiment and then produced by an RF generator. SWIFT excitation also allows to remove ions of predefined m/z ranges from the ICR cell. This results in storage of a small m/z range, or after repeated SWIFT pulsing of a single nominal mass out of a broad band spectrum. Those ions are then accessible for ultrahigh resolution measurements or as precursors for tandem MS. 

In the visible range dye lasers in their various modifications are by far the most widely used types of tunable lasers. Their active media are organic dye molecules solved in liquids, which display strong broad-band fluorescence spectra under excitation by visible or uv light. With different dyes the overall spectral range, where cw or pulsed laser operation has been achieved, extends from 300 nm to 1.2 ym. In this section we briefly summarize the basic physical background and the most important experimental realizations of dye lasers, used in high-resolution spectroscopy. For a more extensive treatment the reader is referred to the laser literature (e.g., [7.31,32b]). 

Emission spectra at these points are shown in Figure 8.2d. The band shapes were independent of the excitation intensity from 0.1 to 2.0 nJ pulse . The spectrum of the anthracene crystal with vibronic structures is ascribed to the fluorescence originating from the free exdton in the crystalline phase [1, 2], while the broad emission spectra of the pyrene microcrystal centered at 470 nm and that of the perylene microcrystal centered at 605 nm are, respectively, ascribed to the self-trapped exciton in the crystalline phase of pyrene and that of the a-type perylene crystal. These spectra clearly show that the femtosecond NIR pulse can produce excited singlet states in these microcrystals. 

---