Two-color Pump Research Articles

Parametric generation of synchronized, independently tunable subpicosecond pulses in the midinfrared with adjustable pulse duration

Intense pulses for two-color pump–probe spectroscopy in the mid-infrared spectral region are generated by optical parametric oscillator (OPO) and optical parametric amplifier (OPA) stages. Synchronous pumping of two OPO’s in parallel delivers tunable pulses in the range 1.2–1.7 μm with a duration of 300 fs. Subsequent frequency conversion in two OPA’s consisting, respectively, of two- and four-crystal setups of AgGaS2 yields pulses in the wavelength interval 2.6–7 μm with a duration of 0.5 ps and single-pulse energy up to 2 μJ. By simple changes of the operating conditions of the two OPO’s, independent adjustments of the resulting mid-infrared pulse duration are achieved that range from 0.5 to 2.6 ps while the pulse duration–bandwidth product of the pulses varies from 0.6 to 1.2.

Temperature Dependence of Excitation Transfer in LH2 of Rhodobacter sphaeroides

Using two-color pump−probe femtosecond spectroscopy, the temperature dependence of the energy transfer rate within the peripheral light-harvesting antenna (LH2) of the photosynthetic bacterium Rhodobacter sphaeroides has been measured. The energy transfer time from B800 to B850 is determined to be 0.7, 1.2, and 1.5 ps at 300, 77, and 4.2 K, respectively. These data, combined with earlier results, have been analyzed with regard to the crystal structure and spectroscopic properties of the purple bacterial LH2 complex. We conclude that the transfer within B800 occurs mainly via the incoherent Förster hopping mechanism. For B800 to B850 transfer, estimates based on the Förster formula yield considerably slower transfer times than experimentally observed, suggesting that an additional mechanism may be involved in enhancing the transfer rate. We suggest two possibilities: transfer via the upper excitonic component of B850 band and/or transfer mediated by a carotenoid molecule.

Excited State Energy Relaxation in the FMO Complexes of the Green BacteriumProsthecochlorisaestuariiat Low Temperatures

Energy transfer processes at 10 K in the water-soluble bacteriochlorophyll a−protein complex (FMO complex) from the green bacterium Prosthecochloris aestuarii have been studied by femtosecond one- and two-color pump−probe measurements. Decay-associated spectra have been obtained for different excitation wavelengths. From these we conclude that energy relaxation occurs via a stepwise, cascading process down the Qy manifold of energy states in the FMO complex. Observed time constants for these steps are 500 fs and 1.7, 5.5, and 30 ps and are correlated with the position in the Qy manifold, becoming shorter with increasing energy. Anisotropy decay functions at different wavelengths exhibit two major components: one of ∼100 fs and one of ∼1 ps.

Vibrational coherence in ultrafast excited state proton transfer

The ultrafast intramolecular proton transfer in the first excited state of 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole in solution was studied in two-color pump probe experiments with 25 fs pulses. The temporally and spectrally resolved stimulated emission of the keto-type reaction product reveals for the first time coherent vibrational motions in the excited state which occur during and after proton transfer. In particular, low frequency modes at 250 and 470 cm −1 show coherent oscillations for times up to 700 fs. Our results suggest that the highly anharmonic in-plane-deformation mode at 470 cm −1 modulates the separation between proton donor and acceptor atom and plays a key role in the transfer reaction.

Two-color midinfrared saturation spectroscopy of intersubband transitions in multiquantum wells

A two-color pump–probe study of bound-to-bound and bound-to-quasibound intersubband transitions in InGaAs/AlGaAs and GaAs/AlGaAs multiquantum wells (MQW), with transition energies near 250 meV has been performed. Electrons were optically excited from the lowest to the upper subband using short midinfrared pulses of intensities up to 108 W/cm2 from a dual optical parametric generator. Differential transmission spectra, taken with the weaker probe pulses, evidence homogeneous line broadening behavior. From the width of the coherence peak, centered near the pump frequency, and arising from a dynamic population grating, the upper state lifetime (bound-to-bound transition) was found to be 0.6 ps, in good quantitative agreement with absorption saturation data.

Transient absorption spectra of vibrationally excited OH/OD groups in mordenite zeolites: Effect of Xe adsorption

Transient spectra following the v=1←0 excitation of the OH stretching mode of Bro/nsted acidic hydroxyl groups in normal and deuterated mordenite zeolites have been measured by a two-color picosecond pump–probe technique. When the OH and OD stretching bands were excited by resonant IR pulses, transient bleaching of the fundamental band (v=1←0) and the transient hot band (v=2←1) absorption signal were observed. For the OH and OD groups free of adsorbates, the spectral widths of the pump-induced signals were broadened by ∼4 cm−1 due to pure dephasing from the convoluted width of the pump and probe IR pulses. When the OH and OD groups were adsorbed by a Xe atom, the frequencies of the ν(OH) and ν(OD) bands shifted to lower frequency and at the same time the widths of both the transient bleaching and the transient hot band absorption were much broader than those of the isolated hydroxyl groups by the factor of more than three. Actually, the widths of the transient signals were about the same as those of the linear absorption signals. An adiabatic approximation has been employed to interpret the observed broadening/shift of the IR bands by making use of the results of DFT (density functional theory) calculations performed on a model cluster consisting of a Xe atom and an H3Si–(OH)–AlH3 substrate. The result agreed well with the observed spectral features.

Time-resolved study of intervalence band thermalization in a GaAs quantum well

The thermalization of optically excited cold holes in a GaAs quantum well is investigated by femtosecond two-color pump–probe measurements. Clear evidence is found for scattering from heavy-holes into the lowest light-hole band due to LO-phonon absorption. We obtain firm data on scattering times which depend strongly on lattice temperature. They vary from 230 fs at room temperature to 900 fs at T=105 K. The experimental data are well reproduced by numerical calculations.

Vibronic excitations of large molecules in solution studied by two-color pump–probe experiments on the 20 fs time scale

The ultrafast vibronic response of organic dye molecules in solution is studied in pump–probe experiments with 30 fs excitation pulses resonant to S0–Sn transitions. The molecular dynamics is probed either by pulses at the same spectral position or by 20 fs pulses overlapping with both the S0–S1 absorption and emission bands. Three contributions on distinctly different time scales are observed in the temporally and spectrally resolved two-color measurements. In the regime below 50 fs, a strong coherent coupling of the S0–Sn and the S0–S1 transitions occurs that is due to coherent vibrational motions in the electronic ground state. This signal is superimposed on the fast bleaching of the electronic ground state, resulting in a steplike increase of transmission. In the range of the S0–S1 emission band, one finds a subsequent picosecond rise of transmission that is due to stimulated emission from vibronic S1 states. The data demonstrate that the relaxation of Sn states directly populated by the pump pulses is much faster than the buildup of stimulated emission. This gives insight into different steps of intramolecular vibronic redistribution and is compared to the Sn–S1 relaxation in other molecules.

Dissipative Exciton Motion in a Chlorophyll a/b Dimer of the Light Harvesting Complex of Photosystem II: Simulation of Pump−Probe Spectra

The ultrafast dissipative exciton motion in a molecular dimer of the light harvesting complex II of higher plants is investigated theoretically. The density matrix formalism is applied to simulate the one- and two-color pump−probe spectra measured by Bittner et al.1 To incorporate coherent vibrational motion we use a representation in localized exciton vibrational states. The observed ultrafast transfer dynamics on the subpicosecond time scale can be well explained by means of a delocalization of the exciton wave function over both monomers. For the low-temperature one-color pump−probe measurement we predict quantum beats in the signal due to coherent nuclear wave packet motion. Furthermore, our calculations support the in-line geometry for the Chlorophyll a/b dimers in the pigment−protein complex.

Spectral, temporal, and spatial characteristics of plasma-induced spectral blue shifting and its application to femtosecond pulse measurement

The spectral, temporal, and spatial characteristics of plasma-induced spectral blue shifting in atmospheric-density rare gases were investigated by a two-color pump–probe technique. When the degree of ionization is less than 1, experiments and theoretical results show that field ionization occurs over a time approximately equal to half the pump-pulse width and that ionization-induced defocusing causes spectral blue shifting to be spatially dependent. The pulse width of a 147-nm pulse generated by four-wave mixing of a femtosecond KrF laser pulse in xenon was determined to be 400 fs by a cross-correlation technique based on plasma defocusing.

Interexciton-State Relaxation and Exciton Localization in Allophycocyanin Trimers

We have employed dynamic absorption, transient hole-burning, and discrete two-color pump−probe femtosecond spectroscopic methods to obtain a time-resolved view of exciton dynamics in trimers of allophycocyanin, the major component of the core of the phycobilisome in cyanobacteria. Allophycocyanin trimers contain a C3-symmetric array of phycocyanobilin (open-chain tetrapyrrole) dimers. The femtosecond time-resolved pump−probe spectra and single-wavelength transients observed in allophycocyanin preparations were interpreted with the aid of a series of calculated spectra, which are based on an assignment of the ground-state absorption spectrum in terms of exciton-coupled chromophore dimers. The observed and calculated time-resolved pump−probe spectra indicate that interexciton-state relaxation, a radiationless transfer of population between dimer exciton states, is responsible for a red shift of the spectra on the ∼30-fs time scale. The time evolution of the pump−probe spectra is inconsistent with a model ac...

Optical dynamics of excitons in J aggregates of a carbocyanine dye

Results of temperature and wavelength dependent fluorescence lifetime and accumulated photon-echo experiments on the J aggregate of the dye 5,5′,6,6′-tetrachloro-1,1′-diethyl-3,3′-di(4–sulfobutyl) -benzimidazolocarbocyanine (TDBC) in an ethylene glycol/water glass are reported. Additionally, the temperature dependent relative fluorescence quantum yield was determined. Using two-color pump–probe spectroscopy, the intersite correlation of the frequency disorder and the size of the coherence domains were estimated. Both the phase and the population relaxation times are frequency dependent in the echo and the single-photon-counting experiments. The dependence of the fluorescence lifetime on detection wavelength is restricted to temperatures below 80 K, indicating a temperature activated process. From our experiments and by comparison with previously published results we conclude that the dispersive nature of both the dephasing and the depopulation is caused by intraband relaxation processes. At higher temperatures this wavelength dependence vanishes due to thermalization. The temperature dependence of the relative fluorescence quantum yield of TDBC is equal to the one of pseudoisocyanine (PIC). Using a motional narrowing model for disordered molecular aggregates with consideration of intersite correlation, at 1.5 K, the two-color pump–probe experiments indicate a very high correlation in the frequency disorder in TDBC J aggregates with a correlation length of several hundred molecules. From pump–probe experiments the delocalization length of the exciton is determined to be between 30 and 45 molecules at 1.5 K.

Local energetic disorder in molecular aggregates probed by the one-exciton to two-exciton transition

We demonstrate a novel approach to probing the magnitude and degree of spatial correlation of local (molecular or atomic) energetic disorder in delocalized exciton systems. The approach is based on measuring the correlation between the ground state to one-exciton and the one-exciton to two-exciton transition frequencies using two-color pump—probe experiments. We apply this technique to aggregates of pseudo-isocyanine and find surprisingly strong intermolecular correlations in the disorder.

Nonlinear optical line shapes of disordered molecular aggregates: Motional narrowing and the effect of intersite correlations

We theoretically investigate nonlinear optical line shapes of linear molecular aggregates with Gaussian disorder in the molecular transition frequencies. A perturbative treatment in the disorder is used, within which the joint stochastic distribution function of the frequencies of all multiexciton states of an aggregate can be determined analytically. It is shown that motional narrowing, which is characteristic for the linear absorption spectra of aggregates, also occurs for nonlinear line shapes. An important aspect of our disorder model is that it allows for general correlations between the transition frequencies of molecules within one aggregate, thereby interpolating between continuous energy disorder and a segment or kink model. The general theory is applicable for nonlinearities of any order. Specific applications are discussed for linear absorption, nonlinear absorption, and two-color pump–probe spectra. Our theory suggests that pump–probe experiments provide a novel and very promising approach to obtain microscopic information on aggregate systems; in particular, this technique can be used to determine both the magnitude of the molecular disorder and its degree of intersite correlation within aggregates.

Vibrational branching ratios from the dissociation of the NeIBr van der Waals molecule

The degree of vibrational excitation in the IBr fragment from the vibrational predissociation of NeIBr (A 3Π1) has been measured using two-color pump–probe laser-induced fluorescence spectroscopy. We find that for the lowest initial vibrational states examined, Δv=−1 dissociation pathways dominate the dynamics, while this channel is closed for v≥17. From this result, the A state binding energy (D0) of the complex is determined to be 67±4 cm−1, while that in the X electronic state is found to be 73±4 cm−1. The X state binding energy is identical to that for NeI2 and NeBr2, suggesting that the potential energy surface for NeIBr can be constructed from a summation of atom–atom pair potentials; we present such a model potential energy surface. The variations in the vibrational branching ratios, when combined with the trends in the predissociation rates, point to the importance of fragment rotational excitation in the dynamics of the dissociation.

SYNCHRONIZATION FO A Q-SWITCHED Nd-YAG PUMPED DYE LASER AND AN EXCIMER LASER FOR TIME-RESOLVED RAMAN EXPERIMENTS

A time resolved resonance Raman experimental set-up is described. It is based on the use of two independent lasers : a Q-switched Nd-YAG pumped dye laser and an excimer laser. A synchronization electronic device has been made to fire the two lasers with variable delays in the range .0 ns -+ 10 ms with a 10 ns step variation. A drift control unit is used to prevent any slow drifts of the excimer laser. The overall jitter between the two laser pulses is 4.5 ns. INTRODUCTION Time resolved resonance Raman spectroscopy is becoming a very powerful technique for the caracterisation of reactive excited states and transient intermediates in fast photochemical reactions (1,2). It provides the means of identifying the transient species, analysing their structures and bonding, and also of monitoring their kinetics of formation and decay. The two-color pump probe method is generally applied. The pump beam is used to start the photochemical reaction by providing energy to the system and promoting the molecules to high excited states. The probe, which is delayed with respect to the pump, is used to excite resonance Rarnan spectra of the transient species. Both have to be tuned to an electric transition : pump to the absorption wavelength of the molecules in the ground state and probe to the transient absorption of the species being analysed. In recent experiments, we have recorded the resonance Raman spectra of excited states and radicals of various molecules using a Q-switched Nd-YAG pumped dye laser (3,4,5). In these cases, the third or the fourth harmonic of the YAG laser was used as the pump beam and the resonance Raman spectra were excited by the dye laser pumped by either the second or the third harmonic of the YAG. The probe beam was delayed by an optical delay line (3 ns/m). Unfortunately, the low quality of the laser beams makes it impossible to use large delays and most experiments were performed in the range 20-50 ns. These experimental conditions did not allow kinetic measurements to be made. In order to overcome this problem, one has to use two independent lasers and synchronize their firing to increase the delays time between the probe and pump beams. TIME-RESOLVED EXPERIMENTS WITH TWO LASERS We have developed a new time-resolved Rarnan experimental set-up using the same Q-switched Nd-YAG pumped dye laser for the probe beam. An excimer laser was preferred for the pump beam because it is less expensive than a second Nd-YAG pumped dye laser and it emits many wavelengths in the UV region (193, 222, 248, Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19917123 C 7 4 6 4 JOURNAL DE PHYSIQUE IV 308, 351 nm) that are generally suitable to fit the absorption spectra of most organic compounds. We have used an YG 581 C QUANTEL YAG laser pumping a TDL 50 QUANTEL dye laser. The output of the dye laser can be continuously tuned from 220 nm to 750 nm by direct pumping of red dyes or coumarin using the second and the third harmonics of the YAG respectively, or by frequency-doubling and/or frequency mixing. The Excimer laser is a QUESTEK model 2420. The problem was to synchronize the two lasers with as low a jitter as possible. In all cases, the jitter time must be lower than the duration of the laser pulses, i.e. 7-9 ns for the dye and 20-30 ns for the Excimer. The repetition rate was fixed at 10 Hz because of the YAG laser. The YAG laser needs three trigger orders to be fired ; one to command the charge of the capacitors, the second to fire the flashlamps and the third to trig the pockels cell. The capacitors require about 80 ms to be charged. The excimer laser needs two trigger orders. One to command the charging of the capacitors and the second to fire the thyratron. For this laser, about 30 ms are needed to charge the capacitors. A digital pulse and delay generator has been designed to generate all the trigger orders, all the delays, and also to synchronize a gated multichannel detector with the YAG laser. The schematic diagram and the chronogram of this system are given in figures 1 and 2.

High-resolution zero kinetic energy electron spectroscopy of ammonia

Rotationally resolved ‘‘zero kinetic energy (ZEKE)’’ electron spectra of ammonia obtained in a 2+1 photon two-color pump–probe experiment are reported. The rovibronic states with J′=3, K′=1 (ortho-NH3) and J′=3, K′=2 (para-NH3) in the B̃ (1E″) v2=2 state are used as intermediate resonances. Rotational energy levels of the NH+3 ion in two vibrational states of the X̃+ (2A″2) electronic ground state, v+2 =1 and 2, are observed in the ZEKE spectra. The rotational constants are: B+=10.19±0.03 cm−1 and C+=5.30±0.04 cm−1 for v+2 =1, and B+ =9.77±0.04 cm−1 and C+ =5.39±0.05 cm−1 for v+2 =2. The adiabatic ionization energies are determined as 83 062.5±1 cm−1 for X̃+ v2=1 and 84 002.9±1 cm−1 for X̃+ v2=2, with respect to the lowest (unoccupied) v2=0+, J″=0, K″=0, Γevr=A1 rotational state of the X̃ (1A′1) electronic ground state of NH3. The observed propensity rules for the change in K quantum number ion←neutral in the rotational ZEKE transitions are explained from ‘‘near symmetry’’conservation rules in electronically allowed transitions.