Picosecond laser pulses, and

In TCSPC, the sample is excited with a picoseconds laser pulse and each single emitted photon is recorded with picoseconds accuracy. The sample is exited at MHz... 

Second, if the intensities of the impinging optical fields are fast and oscillatory (e.g., picosecond laser pulses) and their time durations are comparable to the internal relaxation dynamics of the molecules, those optical fields will see the transient responses of the molecules. These transient responses in the internal motions... 

Noguchi, H., Okada, T, Onda, K., Kano, S. S., Wada, A. and Domen, K. (2003) Time-resolved SFG study of formate on a Ni(lll) surface under irradiation of picosecond laser pulses. Surf. Sci., 528, 183-188. 

The jitter between the laser pulse and the electron pulse was estimated from the measurement using a streak camera (C1370, Hamamatsu Photonics Co. Ltd.), because the jitter is one of important factors that decide the time resolution of the pulse radiolysis. The jitter was several picoseconds. To avoid effects of the jitter on the time resolution, a jitter compensation system was designed [74]. The time interval between the electron pulse (Cerenkov light) and the laser pulse was measured by the streak camera at every shot. The Cerenkov radiation was induced by the electron pulse in air at the end of the beam line. The laser pulse was separated from the analyzing light by a half mirror. The precious time interval could be... 

Picosecond-resolved thermochemical information can be extracted from the evolution of a transient grating produced by the crossing of two laser pulses and interrogated with a third short pulse of light. Several groups have applied this method to thermodynamic questions about the decay of excited states and the evolution of excited states into reactive intermediates. 

An approximate method, described in detail in Ref. (15), was applied to simulate a complementary pump-probe experiment performed with picosecond laser pulses. In this method the interaction with the probe laser pulse is approximated. A complete three-dimensional ab initio simulation, as carried out for the femtosecond experiment, is hardly possible for the picosecond experiment with the computers available today. The free laser pulse parameters were taken from the picosecond experiment duration AffWhm = 1.5 ps, intensity I = 300 MW/cm2, and central wavenumber v = O.QTiEh/h = 16021 cm 1. The dynamics induced by such a laser pulse are illustrated by... 

The 3d ab initio simulations [4] for Na3 are based, in a similar way, on three ab initio potential-energy surfaces for Na3(X), Na3(B), and Na3(X), with 3d ab initio dipole coupling between Na3(X) and Na3(B) evaluated by V. Bonacic-Koutecky et al. [5] plus Condon-type coupling between Na3(B) and Na3(X). Additional potential-energy surfaces interfere at the conical intersections of the pseudo-Jahn-Teller distorted Na3(B) state (see Ref. 6), but we have tested carefully [4] that these interferences are negligible in the frequency domains of the experimental femtosecond/picosecond laser pulse experiments [7] as well as in the continuous-wave experiments [8]. 

The original theory of individual state-selective vibrational transitions induced by IR femtosecond/picosecond laser pulses has been developed by Paramonov and Savva et al. for single laser pulses [13] (see also Ref. 23) followed by more general extensions to series of IR femtosecond/picosecond laser pulses in Refs. 14 and 24. For illustration, let us consider two simple, one-dimensional model systems that are assumed to be decoupled from any... 

The OH and SBV potential-energy surfaces V(q) versus bond or reaction coordinates q, together with the vibrational levels Ev and eigenfunctions 4>v(q), are shown in Figs. 1 and 2, respectively. Our first task will be to design optimal IR femtosecond/picosecond laser pulses for vibrational transitions,... 

Figure 1. Morse potential V(q), vibrational levels Ev, and wave functions < (q) for the model OH (adapted from Ref. 14). The arrows indicate various selective vibrational transitions as well as above-threshold dissociations (ATDs) induced by IR femtosecond/picosecond laser pulses, as discussed in Sections III.A-III.D see Figs. 3-5. Horizontal bars on the arrows mark multiple photon energies ha of the laser pulses cf. Table 1. The resulting ATD spectrum is illustrated by the insert above threshold.

Figure 2. Double-well potential V(q) with corresponding vibrational levels Ev and wave functions v(q) for the model 2,6-dicyanoethylmethylsemibullvalene (SBV) (adapted from Ref. 26). The reaction coordinate q indicates the Cope rearrangement of the model SBV from the reactant (R) isomer versus the transition state 1 to the product (P) isomer. Vertical arrows indicate the laser control of the isomerization R - — P by two IR femtosecond/picosecond laser pulses cf. Fig. 6 and Table I.

These selective transitions (1), (7), and (9) may be achieved by proper optimization of the parameters eo and w, as described elsewhere [13, 18, 21]. Extensions to IR femtosecond/picosecond laser-pulse-induced dissociation or predissociation have been derived in Ref. 16, using either the direct or the indirect solutions of the Schrodinger equation (2) the latter requires extensions of the expansion (5) from bound to continuum states [16,31]. (The consistent derivation in Ref. 16 is based on S. Fliigge in Ref. 31). The same techniques can also be used for IR femtosecond/picosecond laser-pulse-induced isomerization as well as for more complex systems that are two dimensional, three dimensional, and so on, at the expense of increasing numerical efforts due to the higher dimensionality grid representations of the wavepackets f/(t) or the corresponding expansions (5) (see, e.g., Refs. 18, 20, and 21). 

Extensions from the preceding ideal, isolated systems to others that are coupled to an environment are quite demanding and nontrivial [32] because the IR femtosecond/picosecond laser pulse has to achieve the selective vibrational transition (9) in competition against nonselective processes such as dissipation. For simulations, we employ the equation of motion for the reduced-density operator... 

The results are shown in Fig. 3 and Table I. Apparently, optimal IR femtosecond/picosecond laser pulses with durations tp = 500 fs may induce nearly perfect transitions (12), (13) in the model OH. Similar examples are documented in Refs. 13, 18, and 23. A detailed discussion of the derivation of the optimal laser parameters, depending on the vibrational level Ev and the transition dipole matrix elements nvw, is also given in Refs. 13, 18, and 23. Suffice it here to say that in many (but not all) cases the optimal frequency is close (but not identical) to the resonance frequency,... 

Figure 3. Selective vibrational transitions OH(l>, = 0) - OH(ty = 5) and OH(u, = 5)->-OH(iy = 10) induced by two individual IR femtosecond/picosecond laser pulses. The electric fields c(i) and the population dynamics Pv(t) are shown in panels (a) and (b), respectively. Sequential combination of the two individual laser pulses yields the overall transition OH(u = 0) - OH(u = 5) - OH(u/ = 10) cf. Fig. 1 and Table I. For the isolated system, the population of the target state Pv= fo(t) is constant after the series of IR femtosecond/picosecond laser pulses, i > 1 ps.

Figure 5. Series of IR femtosecond/picosecond laser pulses for the sequence of transitions OH(u = 10)- OH(u = 15) - 0 + H for the isolated model OH cf. Fig. 1 and Table I. The notations are as in Fig. 3 populations Pwen(t) = Xoio Po( ) and PCOnt(t) = lPweiKO indicate the total populations of bound and continuum states embedded in the potential well and above the dissociation threshold, respectively. The resulting spectrum of ATD is shown in Fig. 1.

The technique of up-conversion photoluminescence allows one to record the transient PL of a system at the temporal resolution of the laser pulse. It is used to study very fast processes below the picosecond time domain. A typical set-up for this experiment is shown in Fig. 3. The sample is excited at frequency uq by a femtosecond laser pulse and its PL at ujj- is mixed with that of an optically... 

The experiments described above used nanosecond laser pulses, which are much longer than the rotational period of the molecules. At the termination of the pulse, the pendular state that is formed relaxes adiabatically to a free-rotor eigenstate. If instead picosecond laser pulses are used, a rotational wave packet is formed by successive absorption and re-emission of photons during the laser pulse. Such wave packets are expected to display periodic recurrences of the alignment after the end of the pulse. 

Let us mention the rotational beats which have recently been observed (with a trans-stilbene molecule in a gasodynamic jet) and interpreted by Felker and Zewail [54, 145]. The essence of this interesting phenomenon may be described in a simplified way as follows. Let a picosecond laser pulse be capable of coherently exciting from some rotational state J f of a... 

For DFWM and Z-scan with picosecond laser pulses molecular vibrations (or optical phonons in crystals) and reorientation of small molecules can add contributions to the electronic nonlinearity. For longer laser pulses even large molecules can orient and also thermal contributions can occur. 

The detection efficiency of C6H5X (X = F, Cl, Br and I) was investigated with the laser multiphoton ionization method152. The laser-induced ion yield depends mainly on the cross sections of the transitions available to the molecule ground state and on the lifetime of the intermediate electronic state that is initially excited. If a species has a radiative lifetime that is very short compared to the pulse duration, it may relax after excitation and will not be ionized. Molecular ions will therefore be obtained when laser pulses that are at least as short as the excited-state lifetimes are employed. The S excited states of halobenzenes are estimated to have subnanosecond lifetimes, with the exception of fluorobenzene for which a lifetime of the order of 9-10 ns has been calculated at 2ex = 266 nm. Picosecond laser pulses are therefore found effective in producing ionization of halobenzenes with short lifetimes, whereas nanosecond pulses are not152. 

Therefore very narrow excitation pulse widths are necessary, for example, to measure sub-nanosecond relaxation times. A number of methods for generating picosecond laser pulses have been devised and several reviews of these techniques are available [10, 11]. 

---