Femtosecond pump-probe experiments

Femtosecond pump-probe experiments have burgeoned in the last ten years, and this field is now connnonly referred to as laser femtochemistry [26, 27, 28 and 22],... 

Pollard W T, Lee S-Y and Mathies R A 1990 Wavepacket theory of dynamic absorption spectra in femtosecond pump-probe experiments J. Chem. Phys. 92 4012... 

The results for the A state show that a different mechanism is operative. A series of femtosecond pump-probe experiments were performed at wavelengths corresponding to the Rydberg states A (v = 0,1,2) of ammonia molecules.64-66,68,69 The wavelengths used to access these vibrational levels were 214 nm, 211 nm, and 208 nm for the pump laser and 321 nm, 316.5 nm, and 312 nm for the probe laser, respectively. 

Describing complex wave-packet motion on the two coupled potential energy surfaces, this quantity is also of interest since it can be monitored in femtosecond pump-probe experiments [163]. In fact, it has been shown in Ref. 126 employing again the quasi-classical approximation (104) that the time-and frequency-resolved stimulated emission spectrum is nicely reproduced by the PO calculation. Hence vibronic POs may provide a clear and physically appealing interpretation of femtosecond experiments reflecting coherent electron transfer. We note that POs have also been used in semiclassical trace formulas to calculate spectral response functions [3]. 

Importantly, Cl s seems to be involved in many classes of physical, chemical and biological processes, from pericyclic reactions to the complex light harvesting and energy conversion functions of chromophores in proteins (See in this volume) and others amply described in this conference. In contrast, direct experimental information on the passage of the vibrational wavepacket through or near Cl s is less abundant. It mostly concerns, femtosecond pump-probe experiments on isolated organic molecules in the gas phase. 

Experimentally it has proven very difficult to investigate bimolecular reaction kinetics. Although optical techniques have been developed with femtosecond time resolution, the bimolecular nature of the reactions precludes standard femtosecond pump-probe experiments, as a common starting time for the reaction is not readily established. Here we initiate the fast bimolecular acid-base reaction by converting a very weak base, NO3 to a weak base ONOO" and follow the reaction ONOO + H+ as a function of [ft]. 

G. Gerber At moderate laser intensities we do see in femtosecond pump-probe experiments a very similar slow time and long time dynamics in all cluster sizes n > 5 up to n = 50 (largest size investigated up to now) irrespective of the charge state of the particular Hg cluster. From single-pulse TOF mass spectrometry we infer that the... 

There are three quite distinct perturbations that could have caused the vibrational motion to delocalize. The first is that the potential that governs the vibrational motion is not harmonic and it is not harmonic either in the simulations or, of course, in the real molecule. In our experience, under most circumstances this is the effect that comes in at the earliest times. We have seen this to be the case not only for a chemical reaction but also in simulations of femtosecond pump-probe experi-... 

The wavepacket calculation for the femtosecond pump-probe experiment presented in Fig. 16 (bottom) is the result of the first consistent ab initio treatment for three coupled potential-energy surfaces in the complete three-dimensional vibrational space of the Naa molecule. In order to simulate the experimental femtosecond ion signal, the experimental pulse parameters were used duration A/fWhm = 120 fs, intensity I - 520 MW/cm2, and central... 

Fig. 16.1. (a) Schematic illustration of a femtosecond pump-probe experiment. The pump laser with wavelength Ai excites the molecule from the ground-state potential Vo to an excited state-potential V. After a delay r the probe laser with wavelength A2 excites the transient molecule to a second excited-state potential V2. (b) Absorption signal of the transient molecule if the wavelength of the probe laser is tuned to the asymptotic wavelength Ag0 (upper part) or to a wavelength shifted to the red of Ag0 (lower part). Reproduced from Zewail (1988). 

Engel, V. (1991c). Femtosecond pump/probe experiments and ionization The time dependence of the total ion signal, Chem. Phys. Lett. 178, 130. 

In order to apply these equations to a femtosecond pump-probe experiment, an additional assumption has to be made regarding the shape of the time resolved signal. We wish to account for the finite relaxation time of the transient polarisation and so the signal must be described by a double convolution of an exponential decay function with the pump and probe intensity envelope functions. We will assume a Gaussian peak shape so that the convolution may be calculated analytically. As we will see, the experimental results require two such contributions, and hence, the following function will be used to fit the experimental data... 

From a frequency domain point of view, a femtosecond pump-probe experiment, shown schematically in Fig. 1, is a sum of coherent two-photon transition amplitudes constrained by the pump and probe laser bandwidths. The measured signal is proportional to the population in the final state Tf) at the end of the two-pulse sequence. As these two-photon transitions are coherent, we must therefore add the transition amplitudes and then square in order to obtain the probability. As discussed below, the signal contains interferences between all degenerate two-photon transitions. When the time delay between the two laser fields is varied, the... 

Our goals were to elucidate important physical concepts in energy-angle resolved TRPES and to illustrate the range of its applicability to problems in molecular dynamics. We discussed general aspects of femtosecond pump-probe experiments from both the wave packet and the frequency domain point of view. Experimentalists are, in principle, free to choose a final state in which to observe the wave packet dynamics of interest. We emphasized the critical role of the choice of the final state in determining both the experimental technique (e.g., collection of photons or particles) and the information content of an experiment (averaged or state-resolved). The molecular ionization continuum has a rich structure that can act as a template onto which multidimensional wave packet dynamics may be projected. The set of electronic states of the cation are sensitive to both the electronic population... 

Among the best well-known examples of photostability after UV radiation, the ultrafast nonradiative decay observed in DNA/RNA nucleobases, has attracted most of the attention both from experimental and theoretical viewpoints [30], Since the quenched DNA fluorescence in nucleobase monomers at the room temperature was first reported [31] new advances have improved our knowledge on the dynamics of photoexcited DNA. Femtosecond pump-probe experiments in molecular beams have detected multi-exponential decay channels in the femtosecond (fs) and picosecond (ps) timescales for the isolated nucleobases [30, 32-34], The lack of strong solvent effects and similar ultrafast decays obtained for nucleosides and nucleotides suggest that ultrashort lifetimes of nucleobases are intrinsic molecular properties, intimately... 

Ru(bipy)3 + is the prototype of a very large family of MLCT species. In the standard model of the photoprocesses of this compound, a photon excites the molecule to an initial Frank-Condon singlet state, MLCT, that rapidly transforms to a triplet, MLCT, with a quantum yield of near unity. Femtosecond pump probe experiments have established a half-life of about 100 fs for the formation of the triplet state. Recent studies utilizing femtosecond time-resolved fluorescence emission spectroscopy has observed fluorescence emission from the Frank-Condon state itself and the hfetime of this state has been estimated to be 40 15 fs. ... 

An example where insight into the detailed mechanism has been achieved is seen in the work by Woeste s group (Daniel et al., 2003). They combined femtosecond pump-probe experiments, ab initio quantum calculations and wave-packet dynamics simulations in order to decipher the reaction dynamics that underlie the optimal laser fields for producing the parent molecular ion and minimizing fragmentation when CpMn(CO)3 is photoionized (Cp = cyclopentadienyl) ... 

In parallel with these experiments, the adiabatic PESs for the ground, ionic and excited states, together with non-adiabatic couplings, were calculated ab initio. Wave-packet dynamics simulations were then carried out on these smfaces and a detailed mechanism was deduced by comparing the results with the experiments using adaptive optimal control and some further two-colour femtosecond pump-probe experiments. 

F. 6.9 Nanosecond las flash photolysis (see Fig. 6.7) and femtosecond pump-probe experiments (see Fig. 6.8) allow to determine the spectra and the reaction rate of diiodoBODIPY (10) excited states and their reaction with oxygen. Reprinted with permission from [24]... 

For this, various 3d quantum ab initio simulations of the wave packet dynamics in Naa B are presented here and compared to ultrashort laser pump probe experiments. In addition to exact QD calculations, an a > proximate QD method is suggested to simulate the main features of a pump probe spectrum. The simulations provide satisfactory results in comparison to exact QD calculations. By means of these two methods it is possible to reproduce and to explain the different experimental pump probe spectra. The 310 fs oscillation in the femtosecond pump probe experiment [62, 81] can clearly be assigned to the Qs vibration, while the 3ps oscillation of the picosecond pump probe experiment [306, 379] is caused by a slow pseudorotational wave packet motion. 

W.T. Pollard, S.Y. Lee, and R.A. Mathies, Wave Packet Theory of Dynamic Absorption Spectra in Femtosecond Pump-Probe Experiments , J. Chem. Phys. 92, 4012 (1990). 

A.J. Dobbyn and J.M. Hutson, Wavepacket Calculations of Femtosecond Pump-Probe Experiments on the Sodium Trimer , J. Phys. Chem. 98, 11428 (1994). 

---