TIME-RESOLVED TERAHERTZ SPECTROSCOPY OF CONDENSED PHASE REACTIONS

Ultrafast optical Kerr effect and photon echo experiments [1] have shown that there is a correlation between the rates of simple chemical reactions and low frequency (10-100 cm-1) librational modes present in polar solvents and proteins. However, no true microscopic identification of the relevant modes has been made, as these experiments are only sensitive to the equilibrium solvent spectrum. Time-resolved THz pump-probe spectroscopy may be more suitable, as it is in principle sensitive to the non-equilibrium dynamics of the solvent modes. We have developed a system for ultrafast visible/nearUV pump, THz-probe spectroscopy. Pump-probe experiments were performed on the carrier dynamics of photoexcited SI GaAs and the dynamic dipole solvation of two different dyes in polar solution.

Femtosecond pulses are generated by a Coherent Mira/RegA laser system resulting in 120fs pulses at 800nm with 4gJ/pulse at a repetition rate of 250 kHz.Nearly single cycle THz pulses with a 200 fs cycle length, a peak frequency of 50 cm -1 and a usable bandwith from to 85cm-1, are generated and electro-optically detected in ZnTe [2, 3].The experimental setup has two optical delay lines, one for the pump-probe delay and one for the gate delay, i.e., the time delay within the THz probe pulse.At each pump-probe delay, the gate delay line can be scanned and the data Fourier transformed to obtain a transient spectrum.Pump-probe experiments were performed on SI GaAs with the pump centered at 800 or 400nm (see Fig. 1).The creation of carriers by the pump pulse gives rise to a transient absorption over the entire accessible frequency range that can be described by a modified Drude-Lorentz model based on a generalized Langevin equation (see Fig. 1).Pumping SI GaAs at 400 nm forces the 100% 10 0 20 40 60 80 Frequency (em-1)

FIGURE
Transient spectra due to free carrier absorption photoinduced by an 800 nm pump pulse in SI GaAs.The spectra were taken at a pump-probe delay of 20 ps and at pump powers varying from full to 20%.All traces were fitted to a modified Drude-Lorentz absorption model.carriers into the L and X valleys; Relaxation, due to the emission of phonons, back into the main 1-' valley is observed to take place in 1.35 + 0.06 ps.
As model systems for condensed phase chemical reactions, the electron-transfer reaction in betaine [4] and the charge-shift reaction in p-nitroaniline (PNA) were used.Betaine undergoes an electron- transfer reaction in circa ps from S1 back to So in which the dipole moment changes by 9D [5].PNA has a dipole moment that increases by 7.3 D upon photoexcitation [6] and an excited state lifetime of circa 25 ps in m-dichlorobenzene.Figure 2 shows the change in the m- dichlorobenzene far-IR absorption after excitation of PNA at 400 nm [7].In both betaine and PNA an oscillatory component is seen with a frequency of 4.0 :t= 0.7 cm -1 and a decay time of 2.0 + 0.8 ps.The data taken in PNA show a bleach of the ground state absorption that grows with 6.9 + 3.3 ps and that does not decay within the observed time window.Static absorption spectra of betaine and PNA in various The pump is at 400 nm (resonant with S So transition of PNA) and the probe is a THz pulse monitored at its peak for various pump-probe delays.
polar solvents show that there is a small blue shift of the far-IR absorption band due to the presence of the dyes.Hence, the persistent bleach observed in the pump-probe signal in PNA may be due to a semi-permanent rearrangement of the solvent molecules around the solute in its polar excited state.Within the signal-to-noise, no such persistent signal is observed in betaine consistent with its short 4.3 ps excited state lifetime in m-dichlorobenzene [4].We surmise that the oscillatory signal is an impulsive response of the solvent molecules in the first solvation shell to the sudden dipole moment change upon photoexcitation of the solute.

FIGURE 2
FIGURE 2 Transient pump-probe signal taken in p-nitroaniline in m-dichlorobenzene.