Nanosecond

In the previous section we discussed light and matter at equilibrium in a two-level quantum system. For the remainder of this section we will be interested in light and matter which are not at equilibrium. In particular, laser light is completely different from the thennal radiation described at the end of the previous section. In the first place, only one, or a small number of states of the field are occupied, in contrast with the Planck distribution of occupation numbers in thennal radiation. Second, the field state can have a precise phase-, in thennal radiation this phase is assumed to be random. If multiple field states are occupied in a laser they can have a precise phase relationship, something which is achieved in lasers by a teclmique called mode-locking Multiple frequencies with a precise phase relation give rise to laser pulses in time. Nanosecond experiments... 

For fluorescent compounds and for times in die range of a tenth of a nanosecond to a hundred microseconds, two very successftd teclmiques have been used. One is die phase-shift teclmique. In this method the fluorescence is excited by light whose intensity is modulated sinusoidally at a frequency / chosen so its period is not too different from die expected lifetime. The fluorescent light is then also modulated at the same frequency but with a time delay. If the fluorescence decays exponentially, its phase is shifted by an angle A([) which is related to the mean life, i, of the excited state. The relationship is... 

The other coimnon way of measuring nanosecond lifetimes is the time-correlated single-photon counting... 

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

Pibel C D, Sirota E, Brenner J and Dai H L 1998 Nanosecond time-resolved FTIR emission spectroscopy monitoring the energy distribution of highly vibrationally excited molecules during collisional deactivation J. Chem. Phys. 108 1297-300... 

The design of a pulsed EPR spectrometer depends heavily on tlie required pulse lengdi and pulse power which in turn are mainly dictated by the relaxation times of tlie paramagnetic species to be studied, but also by the type of experiment perfomied. When pulses of the order of a few nanoseconds are required (either to compete... 

Porter G and Topp M R 1968 Nanosecond flash photolysis and the absorption spectra of excited singlet states Nature 220 1228-9... 

Simulation runs are typically short (t 10 - 10 MD or MC steps, correspondmg to perhaps a few nanoseconds of real time) compared with the time allowed in laboratory experiments. This means that we need to test whether or not a simulation has reached equilibrium before we can trust the averages calculated in it. Moreover, there is a clear need to subject the simulation averages to a statistical analysis, to make a realistic estimate of the errors. 

Tanigaki K, Ebbesen T W and Kuroshima S 1991 Picosecond and nanosecond studies of the excited state properties of C g Chem. Phys. Lett. 185 189-92... 

The vibrationally excited states of H2-OH have enough energy to decay either to H2 and OH or to cross the barrier to reaction. Time-dependent experiments have been carried out to monitor the non-reactive decay (to H2 + OH), which occurs on a timescale of microseconds for H2-OH but nanoseconds for D2-OH [52, 58]. Analogous experiments have also been carried out for complexes in which the H2 vibration is excited [59]. The reactive decay products have not yet been detected, but it is probably only a matter of time. Even if it proves impossible for H2-OH, there are plenty of other pre-reactive complexes that can be produced. There is little doubt that the spectroscopy of such species will be a rich source of infonnation on reactive potential energy surfaces in the fairly near future. 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

Kummer S, Mais S and Basche T 1995 Measurement of optical dephasing of a single terrylene molecule with nanosecond time resolution J. Chem. Phys. 99 17 078-81... 

Hofrichter J, Sommer J H, Henry E R and Eaton W A 1983 Nanosecond absorption spectroscopy of haemoglobin Proc. Natl Acad. Scl. USA 80 2235-9... 

Figure C3.1.6. Block diagram for nanosecond absorjDtion apparatus using multichannel detection. (From Goldbeck R A and Kliger D S 1993 Adethods Enzymol. 226 147-77.)...

Figure C3.1.7. Time-resolved optical absorjDtion data for the Soret band of photo lysed haemoglobin-CO showing six first-order (or pseudo-first-order) relaxation phases, I-VI, on a logaritlimic time scale extending from nanoseconds to seconds. Relaxations correspond to geminate and diffusive CO rebinding and to intramolecular relaxations of tertiary and quaternary protein stmcture. (From Goldbeck R A, Paquette S J, Bjorling S C and Kliger D S 1996 Biochemistry 35 8628-39.)...

Figure C3.1.13. Experimentai configuration for far-UV nanosecond CD measurements using a frequency-upconverted Ti sapphire iaser as a probe source. Pj and P2 are Mgp2 Rochon poiarizers at cross orientations. SP is a strained transparent piate with about i ° of iinear birefringence for quasi-nuii eiiipsometric CD detection. Prism PMj and the iris Ij seiect the far-UV fourth hannonic of the argon iaser-pumped Ti-sapphire iaser s near-IR fundamentai output to probe the eiiipticity of the sampie. A second iaser beam at 532 nm is used to pump CD...

Lewis J W, Yee G G and Kliger D S 1987 Implementation of an optical multichannel analyzer controller for nanosecond flash photolysis measurements Rev. Sol. Instrum. 58 939-44... 

Yuzawa T, Kate C, George M W and Hamaguchi H O 1994 Nanosecond time-resolved infrared spectroscopy with a dispersive scanning spectrometer Appl. Spectrosc. 48 684-90... 

Varotsis C and Babcock G T 1993 Nanosecond time-resolved resonance Raman spectroscopy/Mef/rods Enzymol. 226 409-31... 

Lewis J W, Tilton R F, Einterz C M, Milder S J, Kuntz I D and Kliger D S 1985 New technique for measuring circular dichroism changes on a nanosecond time scale. Application to (carbonmonoxy)myoglobin and (carbonmonoxy)hemoglobin J. Rhys. Chem. 89 289-94... 

Chen E, Goidbeck R A and Kiiger D S 1997 Nanosecond time-resoived spectroscopy of biomoiecuiar processes Annu. Rev. Biophys. Biomoi. Stmct. 26 325-53... 

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