Pump pulse excitation

In another important experiment (1989) Zewail and his group studied the dissociation of sodium iodide (Nal) Na+ r Na + I. The pump pulse excites the ion pair Na+r which has an equilibrium distance of 2.8 A between... 

An intriguing example which highlights the idea of femtosecond chemistry is the photodissociation of Nal (Rose, Rosker, and Zewail 1988 Rosker, Rose, and Zewail 1988). Figure 16.6 illustrates the potential energy curves involved in the fragmentation. The pump pulse excites Nal to a covalent state which, in the diabatic picture, correlates with Na +... 

Two ultrafast electron diffraction studies on the Fe(CO)5 system have been published [69, 70], In these experiments fs pump pulses excite Fe(CO)5 in the gas phase in a free expansion jet. Two photon excitation was used in these experiments... 

When an ultrashort laser pulse is used as the light source for the SHG spectroscopy, the signal contains dynamical information of the system. In time-resolved SHG spectroscopy, two ultrashort pulses with tunable relative time-delay irradiate the sample. The first pulse (pump pulse) excites the system, and the SHG intensity or its spectra induced by the second pulse (probe pulse) are measured as a function of the time delay. 

A similar solvation dynamics of electrons in an ice layer on Cu(lll) has been reported by Gahl et al. [29]. In this case, a pump pulse excites electrons from the substrate into a conduction band of an ice film of four bilayer thickness. This initially delocalized electron is subsequently trapped by a localized state within 100 fs, which leads to a pronounced flattening of the dispersion in the 2PPE spectra. The localized electron further undergoes solvation on a picosecond time scale, which manifests itself experimentally as a shift in the binding energy. 

The conceptionally simplest approach for IR-driven HT, the pump-dump scheme [45], can be illustrated using a one-dimensional double minimum potential as shown in Fig. 4.1. First we notice that the potential is asymmetric which allows one to distinguish between the initial state S and the final state S f of the HT reaction. Initially a pump-pulse excites the system from the localized ground state S to a delocalized intermediate state which usually is energetically above the reaction barrier. From there a second pulse dumps the system into the product... 

Figure 4.1 Illustration of the pump-dump scheme for a slightly asymmetric one-dimensional double minimum potential. In a first step a pump pulse excites the system from the initial state an above-the-barrier state From there a second pulse dumps the population into the target state Pf.

Alternatively, one can take advantage of the developments in ultrafast laser technology and use femtosecond lasers to follow the course of a reaction in real time. In this approach, pioneered by Zewail [36], a unimolecular or bimolecular reaction is initiated by a femtosecond pmnp pulse, and a femtosecond probe pulse monitors some aspect of the reaction as a function of pmnp-probe delay time. The first example of such an experiment was the photodissociation of ICN [37]. Here the pump pulse excited ICN to a repulsive... 

Fig. 13. Schematic view of the irradiated STIRAP process (Stimulated Raman pumping) (a) The chirped pump pulse excites the system the electronic ground state (b) the system is stimulated back down to the electronic ground state, but a Stokes-shifted higher vibrational level, (c) The resulting wave packet is probed by the delayed probe pulse, which ionizes the particle from the oscillating ground state.

When these wavelengths superimpose in phase they are said to be mode-locked and behave as a wave packet or pulse. The pump and probe pulses are focused into a chamber containing gaseous molecules for study. The pump pulse excites a change (e.g., chemical reaction) while absorption of the probe pulse monitors the course of structural change as time passes on the fs scale. Zewail has likened the process to the effects of a strobe light that furnishes stop-action pictures of a fast process. 

Fig. 2 Experimental arrangement for time-resolved FSRS (femtosecond stimulated raman spectroscopy). The femtosecond actinic pump pulse excites the sample electronically. After a delay the femtosecond probe pulse and picosecond Raman pump pulse arrive together to interrogate the instantaneous molecular structure. The self-heterodyned signal is emitted in the probe direction, dispersed, and detected by a kHz readout CCD. Data collection is best performed by division of subsequent Raman pump-on by Raman pump-off laser shots (lower trace), however this has been performed by other groups as a subtraction of subsequent pulses (upper trace). Reproduced from ref 2 with permission from the PCCP Owner Societies (2012).

Ultrafast (femtosecond) pulsed two-color mid-infrared spectroscopy was used by Bakker et al. in a series of papers to study the effect of ions on the structural dynamics of their aqueous solutions as recently reviewed (Bakker 2008). The first intense pulse (pump pulse) excites the O-H (or O-D) stretch vibration to the first excited state and the second pulse (probe pulse), red-shifted with respect to the first, probes the decay of this state. This technique has been applied to aqueous (0.1-0.5 M HDO in D2O) solutions of LiX, NaX, and MgX2 (X = Cl, Br, I), KF, NaC104, and Mg(C104)2 over wide concentration ranges, 0.5-10 mol dm. ... 

In a femtosecond pump-and-probe experiment the pump pulse excites the vibrational levels v = 10-12 in the A X y state of Na2 coherently. The vibrational spacings are 109 cm and 108 cm . The probe laser pulse excites the molecules only from the inner turning point into a higher Rydberg state. If the fluorescence /pKAr) from this state is observed as function of the delay time At between the pump and probe pulses, calculate the period ATi of the oscillating signal and the period AT2 of its modulated envelope. 

FIG.l Normalized absorbance difference spectra of BChl-a in 1-propanol (500 shots) measured at 0, 1, 10, 50 and 150 ps with respect to the pump pulse. (Excited at 607 nm)... 

Fig. 5.3 Pump-pulse excited wavepacket motion on the diabatic states, (a) Vj (ionic) and (b) V2 (covalent). (Reprinted with permission from Y. Arasaki et al, 119, 7913 (2003)).

Figure 5.14(a) shows the time dependence of the diabatic state 1 population Pi t) for various delay times to of the control pulse between 30 and 120 fs. In each panel, the black curve shows P t) computed without application of the control pulse. The curve shows a drop in population between —20 to 20 fs this is the pump pulse excitation. The diabatic representation and the significant V 2 (R) around the initial wavefunction position results in the oscillatory features on top of the population drop seen in this time range (these oscillations are not seen in the adiabatic representation). Then the pump-excited state decrease by 13% (from P2 t) = 0.31 to 0.27) around t = 110 fs this is due to the static crossing at i cross- Without the control pulse, this is the only population transfer seen. 

The curves in the upper panels of Fig. 5.14(a) show P t) under control pulses with various to. The pump pulse excitation (t < 20 fs) is common to all the cases, and the control pulse affects the dynamics after the pump. We see the final population (Pi t) at t = 250 fs) increase from to = 30 to 60 fs, reach the maximum around 70 fs, then decrease as to is made larger, and become even less than the final population found for the case without control by to > 100 fs. For to > 120 fs, the effect of the control pulse becomes smaller, and is completely without effect for to > 140 fs (not shown in Figure). The initial rise in Pi (t) increases as to is increased from 30 to 65 fs, and then decreases as to is increased from 65 to 120 fs. For to < 80 fs, we see an immediate drop in Pi(t) following the initial rise. Then after some time (t > 100 fs), we see the population increase at Pcross that is also present without the control pulse. For 80 < to < 100 fs, we instead see a slow but lasting decrease some time after the initial rise. 

The center of the piunp pulse is taken as time t = 0. The pump pulse excites almost half of the initial groimd state wavefunction on to the excited electronic state. The excited wavepacket immediately starts to move towards the conical intersection/bottom of the excited state, even during the pumping, to first reach the conical intersection by t = 5 fs. By t = 50 fs, the first dissociating components reach the bond distance of 2.1 A, beyond which we consider the molecule dissociated. 

The influence of the rotational motion T ot 18 ps, rotational revival 7"rot,rev > 500 ps, estimated from [323]) can be neglected in the simulation, because the investigations concentrate on the short-time dynamics of the ISC process immediately after pump pulse excitation. The time-dependent evolution of the wave packets is evaluated by solving a set of coupled time-dependent Schrodinger equations... 

Fig. 3.19. Electronic population in A and b 77u states during and after pump pulse excitation for the isotopes (a) and (b) for a propagation time...

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