Methanol laser spectroscopy

Whenbenzophenone is illuminated with ultraviolet radiation, it is excited into a singlet state. This singlet changes rapidly into a triplet, which phosphoresces. Triethylamine acts as a quencher for the triplet. In an experiment in methanol as solvent, the phosphorescence intensity fp, varied with amine concentration as shown below. A time-resolved laser spectroscopy experiment had also shown that the half-life of the fluorescence in the absence of quencher is 29 ps. What is the value of kq ... 

More recently, Scaiano et al. (1991) observed (Zs)->(Z)-isomerization of 1,3-di-phenyltriazene also in methanol by using flash photolysis, transient spectroscopy, and laser-induced optoacoustic calorimetry (LIOAC). The interpretation of the data is consistent with the mechanism shown in Scheme 13-4, involving two solvent molecules. 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

The present study shows a combined investigation of both the generation process of solvated electrons and of pump-probe-spectroscopy of intermediate states including the final equilibrated ground state. As shown previously [5], the formation of the solvated electron in methanol takes place within -10 ps. About an order of magnitude slower than in water, the timescale of this process allows to investigate the precursor states in much more detail using a state-of-the-art laser system. 

The UV-visible absorption and fluorescence spectra of NKX-2311 in solution are measured. The absorption and fluorescence maxima are located at 490 and 555 nm, respectively. The fluorescence spectrum is shown in Fig. 1. This figure also shows the transient absorption spectrum of NKX-2311 in deuterated methanol (CD3OD) solution measured by the nanosecond laser system. The spectrum is ascribed to the excited singlet state (dye ). The absorption spectrum of the oxidized form of NKX-2311 (dye+) was measured by nanosecond transient absorption spectroscopy after adding electron acceptor, 1,4-benzoquinone, in NKX-2311 solution of CD3OD. As shown in Fig. 1, there are two characteristic peaks at around 875 and 1010 nm. 

Laser-flash photolysis of (203) in solution affords the ketene (204) and this is the first time that this ketene has been observed using time-resolved IR spectroscopy, Although other workers have previously reported the formation of this species. The present work reports the kinetics of the reaction of the ketene with water, methanol and diethylamine. The products obtained from the irradiation of a series of AT-acetoacetyl-a-amino acids have been identified by gas chromatography and chemical ionization mass spectrometry. ... 

Diphenylcarbene (diphenylmethylene) can be generated from diphenyldiazomethane (373) by direct irradiation or by triplet sensitization.1156 The intermediate multiplicity then controls the subsequent reactions the singlet carbene inserts into the O—H bond of methanol, whereas the triplet carbene adds to an a I Irene (Scheme 6.173). It has been found that singlet and triplet diphenylcarbenes are in rapid equilibrium relative to the rates of reactions.1157,1158 Competitive quenching experiments (to obtain k1 and kTS) and laser flash spectroscopy (Section 3.7 to obtain k2 and kST) allowed the determination of the free energy difference between the singlet and triplet states of carbene ( 20kJ mol J). 

These interpretations were supported by time-resolved spectroscopy. Laser flash photolysis of 4 in CH2C12 produces a transient spectrum (Figure 11) very similar to that obtained by flash photolysis of phenyl azide (Figure 8) and is therefore attributed to pentafluorodehydroazepine. Similar results were obtained in acetonitrile and tetrahydrofuran. However LFP of 4 in methanol gives an entirely different transient spectrum. (Figure 12). The... 

St -> Sn Spectra.—A description has been given of a method for recording ultrafast absorption spectra using a passively mode-locked ruby laser with a ruby amplifier, a pulsed flashlamp probe source, and streak-camera detection for ps time resolution. Results for the dye 3,3 -diethylthiatricarbocyanine in methanol were reported.2870 These results can be compared with those obtained by an alternative method 29711 which permits nm spectral resolution and ps time resolution over the entire visible region, and which was first used on the Sx -> Sn absorption of 3,3 -diethyloxadicarbocyanine iodide, and which has recently been used to record the Si - Sn absorption spectra of bis-(4-dimethylaminodithio-benzil) nickel(n), and of SnIV, Pd11, and Cu" porphyrins.298 The use of time-resolved Si - Sn, Ti - Tn absorption and emission spectroscopy to assist in the selection of laser dyes has been illustrated with respect to anthracene and its derivatives.299 Si - Sn Spectra of coronene, 1 2-benzanthracene, l 12-benz-perylene, 1,2,3,4-dibenzanthracene, and benzo[6]chrysene in poly(methyl methacrylate) and toluene have been reported, the method of detection being modulation spectrophotometry, for which it is claimed that species of lifetime down to... 

Figure 9 (A) Spectral stripping of components from the tunable diode laser infrared spectrum of cigarette smoke. From bottom to top ethylene, acrolein, methanol, and comparison with hydrazine. The reference spectrum is the lower of the two traces in each case. (B) Expanded view of the residual spectrum compared to that of hydrazine. (Reprinted with permission from Plunkett S, Parrish ME, Shafer KH, Nelson D, Shorter J, and Zahniser M (2001) Time-resolved analysis of cigarette combustion gases using a dual infrared tunable diode infrared laser system. Vibrational Spectroscopy 27 53-63 Elsevier.)...

Both the 9-methyl and 9-trifluoromethyl-9-fluorenyl cations (7 and 8) have been generated by laser flash photolysis of corresponding alcohol precursors. The product ions were then studied by time-resolved spectroscopy. Consistent with previous studies related to carbocation-bearing electron-withdrawing groups, ion (8) exhibits a significant bathochromic shift in the UV absorption compared to (7). The ions were quenched with methanol and reaction rates showed cation (8) to be a more reactive species. 

---