Excitation wavelength and fluence dependent femtosecond transient absorption studies on electron dynamics of gold nanorods

The electron dynamics of gold nanorods were systematically studied by using femtosecond transient absorption experiments. Two different excitation wavelengths (400 and 800 nm) have been used as the excitation sources to selectively excite transverse and longitudinal modes. The transient absorption spectra were found to be strongly dependent on the excitation wavelength and fluence. Laser pulses of 800 nm excite the longitudinal mode directly, which cause an increase in the electronic temperatures and subsequent broadening and bleaching of both the longitudinal and transverse modes. Pulses of 400 nm excite both the transverse and longitudinal modes simultaneously. At low excitation fluences, the energy is distributed into two modes according to their steady state extinction coefficients, under which the transient spectra are similar to those under excitation at 800 nm. However, as the excitation fluence exceeds a threshold, the bleaching of the longitudinal plasmon band saturates and the input energies mainly flow to the transverse mode. As a result, the bleaching of the transverse mode increases rapidly. The electron-phonon dynamics show a strong correlation with the bleaching amplitude. We have tried to explain the results with a consistent picture: the bleaching amplitude and electron-phonon relaxation time are directly related to energy distribution into different modes, which are excitation wavelength and fluence dependent. Our studies help to clarify the seemingly inconsistent results in the previous studies by different research groups.     

Mode-selective excitation of coherent surface phonons on alkali-covered metal surfaces

We demonstrate the mode-selective excitation of coherent phonons at Pt(111) surfaces covered with submonolayer caesium atoms. A burst of 150 fs laser pulses with the repetition rate of 2.0-2.9 THz was synthesized by using a spatial-light modulator, and used for the coherent surface phonon excitation. The coherent nuclear motion was monitored by time-resolved second harmonic generation. By tuning the repetition rate, we succeeded in controlling the relative amplitude of the vibrational coherence of the Cs-Pt stretching mode (2.3-2.4 THz) to that of the Pt surface Rayleigh phonon mode (2.6 or 2.9 THz, depending on the Cs coverage).     

Instantaneous vibrational frequencies of diffusing and desorbing adsorbates: CO/Pt(111)

Electronic excitation of metal by intense laser pulses stimulates nuclear motions of adsorbates through nonadiabatic coupling, resulting in diffusion and desorption of adsorbates. These processes take place via precursor states: adsorbates whose vibrational modes with respect to substrate are highly excited. This paper reports the dynamics of precursor states of CO on Pt(111) probed by use of infrared-visible sum frequency generation with phase-sensitive detection, which allows us to obtain the second-order nonlinear susceptibility and thus the vibrational response function. Without pump pulses at 400 nm, the inverse Fourier transformation of the vibrational response function reveals a free induction decay of vibrational polarization of C-O stretching created by a short infrared pulse. The free induction decay is perturbed when an intense 400-nm pump pulse following the infrared pulse is irradiated, because diffusion and desorption of CO are induced by the pump pulse. The time evolution of instantaneous C-O stretching frequency retrieved from the perturbed free induction decay shows a redshift followed by a rapid reverse shift when the fluence of pump pulse is high enough to desorb CO. This indicates that the frustrated modes of CO is first substantially excited and then the parallel momentum of CO is converted to the normal one through mutual collisions, leading to substantial excitation of the external stretching mode of CO.     

Ultrafast studies of gold, nickel, and palladium nanorods

Steady state and ultrafast transient absorption studies have been carried out for gold, nickel, and palladium high aspect ratio nanorods. For each metal, nanorods were fabricated by electrochemical deposition into approximately 6 microm thick polycarbonate templates. Two nominal pore diameters(10 and 30 nm, resulting in nanorod diameters of about 40 and 60 nm, respectively) were used, yielding nanorods with high aspect ratios (>25). Static spectra of nanorods of all three metals reveal both a longitudinal surface plasmon resonance (SPR(L)) band in the mid-infrared as well as a transverse band in the visible for the gold and larger diameter nickel and palladium nanorods. The appearance of SPR(L) bands in the infrared for high aspect ratio metal nanorods and the trends in their maxima for the different aspect ratios and metals are consistent with calculations based on the Gans theory. For the gold and nickel samples, time resolved studies were performed with a subpicosecond resolution using 400 nm excitation and a wide range of probe wavelengths from the visible to the mid-IR as well as for infrared excitation (near 2000 cm(-1)) probed at 800 nm. The dynamics observed for nanorods of both metals and both diameters include transients due to electron-phonon coupling and impulsively excited coherent acoustic breathing mode oscillations, which are similar to those previously reported for spherical and smaller rod-shaped gold nanoparticles. The dynamics we observe are the same within the experimental uncertainty for 400 nm and infrared (5 microm) excitation probed at 800 nm. The transient absorption using 400 nm excitation and 800 nm probe pulses of the palladium nanorods also reveal coherent acoustic oscillations. The results demonstrate that the dynamics for high aspect ratio metal nanorods are similar to those for smaller nanoparticles.     

Ultrafast nonradiative dynamics in electronically excited hexafluorobenzene by femtosecond time-resolved mass spectrometry

The fast nonradiative decay dynamics of the lowest two excited pipi(*) electronic states (S(2) and S(3)) of hexafluorobenzene have been investigated by using femtosecond time-resolved time-of-flight mass spectrometry. The molecules were excited at wavelengths between 265 nm > or = lambda(pump) > or = 217 nm and probed by four- and three-photon ionization at lambda(probe)=775 nm. The observed temporal profiles exhibit two exponential decay times (tau(1)=0.54-0.1 ps and tau(2)=493-4.67 ps, depending on the excitation wavelength) and a superimposed coherent oscillation with vibrational frequency nu(osc)=97 cm(-1) and damping time tau(D) that is two to three times longer than the respective tau(1). The first decay component (tau(1)) is assigned to rapid radiationless transfer from the excited optically bright pipi(*) electronic state (S(2) or S(3), respectively) through a conical intersection (CI) to the lower-lying optically dark pisigma(*) state (S(1)) of the molecule; the second component (tau(2)) is attributed to the subsequent slower relaxation from the S(1) state back to the electronic ground state (S(0)). tau(2) dramatically decreases with increasing vibronic excitation energy up to the CI connecting the pisigma(*) with the S(0) state. The coherent oscillation is identified as nuclear motion along the out-of-plane vibration nu(16a) (notation as for benzene), which has e(2u) symmetry and acts as coupling mode between the pipi(*) and pisigma(*) states.     

Transient absorption studies of vibrational relaxation and photophysics of Prussian blue and ruthenium purple nanoparticles

Transient infrared and visible absorption studies have been used to characterize vibrational and electronic dynamics of Prussian blue (PB) and ruthenium purple (RP) nanoparticles produced and characterized in AOT reverse micelles. Studies include excitation and probing with both infrared (near 2000 cm(-1)) and visible (800 nm) pulses. From IR pump-IR probe measurements of the CN stretching bands, vibrational population lifetimes are determined to be 32 ± 4 ps for PB and 44 ± 14 ps for RP. These times are longer than those for ferrocyanide (4 ps) and ruthenocyanide (4 ps) in normal water and are closer to the times for these species in heavy water (25-30 ps) and for ferrocyanide in formamide (43 ps). The PB and RP lifetimes are also longer than those (<15 ps) previously measured for CN stretching bands following intervalence excitation and back-electron transfer (BET) for dinuclear mixed-valence compounds containing Fe, Ru, and Os in heavy water and formamide [A. V. Tivansky, C. F. Wang, and G. C. Walker, J. Phys. Chem. A 107, 9051 (2003)]. In 800 nm pump-IR probe experiments on RP and PB, transient IR spectra and decay curves are similar to those with IR excitation; a ground state bleach and a red shifted (by ~40 cm(-1)) excited state decay are observed. These results for the visible pumping are consistent with rapid (<1 ps) BET resulting in population in the ground electronic state with vibrational excitation in the CN mode. In addition, transient absorption studies were performed for PB and RP probing with visible light using both visible and IR excitation. The early time response for the 800 nm pump-800 nm probe of PB exhibits an instrument-limited, subpicosecond bleach followed by an absorption, which is consistent with the previously reported results [D. C. Arnett, P. Vohringer, and N. F. Scherer, J. Am. Chem. Soc. 117, 12262 (1995)]. The absorption exhibits a biexponential decay with decay times of 9 and 185 ps, which could have been attributed to the CN band excitation indicated from 800 pump-IR probe results. However, IR pump-800 nm probe studies reveal that excitation of the CN band directly results in a decreased visible absorption that decays in 18 ps rather than an induced absorption at 800 nm. Characteristics of the IR pump-800 nm probe, especially that it induces a bleach instead of an absorption, unambiguously indicate that the transient absorption from the 800 nm pump-800 nm probe is dominated by states other than the CN bands and is attributed to population in other, probably lower frequency, vibrational modes excited following visible excitation and rapid BET.     

Coherent polyatomic dynamics studied by femtosecond time-resolved photoelectron spectroscopy: dissociation of vibrationally excited CS2 in the 6s and 4d Rydberg states

The dissociation dynamics of the 6s and 4d Rydberg states of carbon disulfide (CS(2)*) are studied by time-resolved photoelectron spectroscopy. The CS(2) is excited by two photons of 267 nm (pump) to the 6s and 4d Rydberg states and probed by ionization with either 800 or 400 nm. The experiments can distinguish and successfully track the time dynamics of both spin [1/2] (upper) and [3/2] (lower) cores of the excited Rydberg states, which are split by 60 meV, by measuring the outgoing electron kinetic energies. Multiple mode vibrational wave packets are created within the Rydberg states and observed through recurrence interferences in the final ion state. Fourier transformation of the temporal response directly reveals the coherent population of several electronic states and vibrational modes. The composition of the wave packet is varied experimentally by tuning the excitation frequency to particular resonances between 264 and 270 nm. The work presented here shows that the decay time of the spin components exhibits sensitivity to the electronic and vibrational states accessed in the pump step. Population of the bending mode results in an excited state lifetime of as little as 530 fs, as compared to a several picosecond lifetime observed for the electronic origin bands. Experiments that probe the neutral state dynamics with 400 nm reveal a possible vibrationally mediated evolution of the wave packet to a different Franck-Condon window as a consequence of Renner-Teller splitting. Upon bending, symmetry lowering from D(infinityh) to C(2v) enables ionization to the CS(2) (+) (B (2)Pi(u)) final state. The dissociation dynamics observed are highly mode specific, as revealed by the frequency and temporal domain analysis of the photoelectron spectra.     

Ultrafast processes in OClO molecules excited by femtosecond laser pulses at 386-409 nm

Ultrafast dissociation dynamics in OClO molecules is studied, induced by femtosecond laser pulses in the wavelength region from 386 to 409 nm, i.e., within the wide absorption band to the (approximately)A (2)A(2) electronic state. The decay of the initially excited state due to nonadiabatic coupling to the close lying (2)A(1) and (2)B(2) electronic states proceeds with a time constant increasing from 4.6 ps at 386 nm to 30 ps at 408.5 nm. Dissociation of the OClO molecule occurs after internal conversion within about 250 fs. In addition, a minor channel of direct excitation of the (2)A(1) electronic state has been identified, the lifetime of which increases from a few 100 fs at 386 nm to 2.2 ps at 408.5 nm. Simultaneous excitation of two neighboring vibrational bands in the (approximately)A (2)A(2) state leads to a coherent oscillation of the parent ion signal with the frequency difference of both modes.     

Stark-assisted population control of coherent CS(2) 4f and 5p Rydberg wave packets studied by femtosecond time-resolved photoelectron spectroscopy

A two-color (3+1(')) pump-probe scheme is employed to investigate Rydberg wave packet dynamics in carbon disulfide (CS(2) (*)). The state superpositions are created within the 4f and 5p Rydberg manifolds by three photons of the 400 nm pump pulse, and their temporal evolution is monitored with femtosecond time-resolved photoelectron spectroscopy using an 800 nm ionizing probe pulse. The coherent behavior of the non-stationary superpositions are observed through wavepacket revivals upon ionization to either the upper (12) or lower (32) spin-orbit components of CS(2) (+). The results show clearly that the composition of the wavepacket can be efficiently controlled by the power density of the excitation pulse over a range from 500 GWcm(2) to 10 TWcm(2). The results are consistent with the anticipated ac-Stark shift for 400 nm light and demonstrate an effective method for population control in molecular systems. Moreover, it is shown that Rydberg wavepackets can be formed in CS(2) with excitation power densities up to 10 TWcm(2) without significant fragmentation. The exponential 1e population decay (T(1)) of specific excited Rydberg states are recovered by analysis of the coherent part of the signal. The dissociation lifetimes of these states are typically 1.5 ps. However, a region exhibiting a more rapid decay ( approximately 800 fs) is observed for states residing in the energy range of 74 450-74 550 cm(-1), suggestive of an enhanced surface crossing in this region.     

Wavelength-independent ultrafast dynamics and coherent oscillation of a metal–carbon stretch vibration in photodissociation of Cr(CO)6 in the region of 270–345 nm

In the group-6 metal hexacarbonyls a number of metal-to-ligand charge-transfer (MLCT) and ligand-field (LF or d → d) states can be excited in the near UV. The latter are repulsive. In equilibrium geometry, most of them are higher than the MLCT states. We probed the dynamics of photodissociation of M(CO)₆ → M(CO)₅ + CO (M = Cr; some data also for M = Mo) with improved time resolution (10–40 fs), pumping at different wavelengths (mainly 270–345 nm) and probing by nonresonant photoionization. The initial relaxation (e.g. within 12.5 fs from T_1u excited at 270 nm) is assigned to direct crossing over to the repulsive surface, from where the subsequent dissociation is also remarkably fast (18 fs in this example). That is, there is no detour via the lowest excited singlet state, in contrast to the usual assumption. Also with 318 and 345 nm excitation a direct MLCT → LF relaxation seems to occur before dissociation. The product M(CO)₅ is generated in the S₁ state, also at pump wavelengths (345 nm) with barely sufficient energy. It relaxes to S₀ through a Jahn–Teller induced conical intersection along pseudorotation coordinates, which stimulates a coherent oscillation in S₀ in this vibration. A higher-frequency oscillation, assigned to totally symmetric MC stretch vibrations, is already found in the Franck–Condon region; it persists (with different wavenumbers) also during dissociation and over the subsequent product states. This vibration is transverse to the valley of dissociation, which is barrierless. The wavelength-independent mechanism also implies that there is no triplet contribution (which was previously supposed at long wavelengths) to photochemical dissociation of the hexacarbonyls.     

Vibrational spectroscopy of selected minerals of the rosasite group

Minerals in the rosasite group namely rosasite, glaucosphaerite, kolwezite, mcguinnessite have been studied by a combination of infrared and Raman spectroscopy. The spectral patterns for the minerals rosasite, glaucosphaerite, kolwezite and mcguinnessite are similar to that of malachite implying the molecular structure is similar to malachite. A comparison is made with the spectrum of malachite. The rosasite mineral group is characterised by two OH stretching vibrations at approximately 3401 and 3311 cm-1. Two intense bands observed at approximately 1096 and 1046 cm-1 are assigned to nu1(CO3)2- symmetric stretching vibration and the delta OH deformation mode. Multiple bands are found in the 800-900 and 650-750 cm-1 regions attributed to the nu2 and nu4 bending modes confirming the symmetry reduction of the carbonate anion in the rosasite mineral group as C2v or Cs. A band at approximately 560 cm-1 is assigned to a CuO stretching mode.     

Coherent oscillation and ultrafast internal conversion of tetrafluoroethene after excitation at 197 nm.

C2F4 was excited by using a 150 fs pulse in its longest-wavelength band to the Rydberg (3 s) state and then probed by photoionization techniques at 810 nm. The molecule relaxes in two consecutive steps (time constants 29 and 118 fs), probably via the pipi* state, which is lowered in energy by stretching and twisting the C=C bond. A coherent oscillation (350 fs) was found, which we assign to an overtone of the twist vibration (47.6 cm(-1)) in this state. we also conclude that dissociation to singlet and some triplet CF2 only takes place in the hot ground state of C2F4, from where also the C2F4 triplet state is populated. The potentials and their conical intersections are discussed with respect to relaxation and dissociation, including also some new considerations of thermal processes.     

Ultrafast excited state dynamics of modified phthalocyanines: p-HPcZn and p-HPcCo

Two modified metallophthalocyanines (MPcs) containing sulfonic naphthoxy substituents were synthesized. The measurements of transient absorption and time-resolved photoluminescence were used to study the ultrafast response and excited state dynamics of two MPcs in dimethyl sulfoxide (DMSO) solution, which were predominantly in the monomeric form. Under excitation at 400 nm, these molecules experience vibrational relaxation to the bottom of the first excited state and then the excitation rapidly converts to the low-lying charge-transfer (CT) state and finally reaches the triplet states. Under excitation at 800 nm, they show a two-photon absorption character, and their excited state dynamics exhibit strong dependence on the probe wavelength. The main results with 400 nm pumping are similar to the results with 800 nm pumping. For p-HPcZn, weak two-photon photoluminescence was also observed with a lifetime of 52 +/- 2 ps. A four-level model was used to illustrate the excited state dynamics of p-HPcZn, while a five-level model was suggested for p-HPcCo molecule.     

The ultraviolet spectrum of the CoCl2 radical, studied at vibrational and rotational resolution

The laser excitation spectrum of the 327 nm band system of CoCl2, formed in a free-jet expansion, has been recorded at a rotational temperature of approximately 10 K. The spectrum is congested and suffers extensive perturbations. A progression in the excited state symmetric stretching vibration has been identified. The decrease in the symmetric stretching vibrational wave number on excitation is considerable [nu1 '=195.7(12), nu1 (")=358.1(17) cm(-1)]. Despite widespread perturbations in the rotational structure of these vibronic bands, they can be confidently assigned to a parallel Omega=72-72 transition, consistent with an inverted 4Deltag ground electronic state. The rotational constant for Co35Cl2 in the ground state is determined to be 0.056 65(11) cm(-1), which corresponds to a value for the zero-point averaged Co-Cl bond length r0 of 2.062 8(40) A. The perturbations are found to be strongly isotopomer dependent.     

High-resolution Absorption Spectra of Acetylene in 142.8-152.3 nm

The absorption spectra of acetylene molecules was measured under jet-cooled conditions in the wavelength range of 142.8-152.3 nm,with a tunable and highly resolved vacuum ultraviolet (VUV) laser generated by two-photon resonant four wave difference frequency mixing processes. Due to the sufficient vibrational and rotational cooling effect of the molecular beam and the higher resolution VUV laser, the observed absorption spectra exhibit more distinct spectral features than the previous works measured at room temperature. The major three vibrational bands are assigned as a CC symmetry stretching vibrational progress (v2=0-2) of the C1Ⅱu state of acetylene.The observed shoulder peak at 148.2 nm is assigned to the first overtone band of the trans-bending mode v4 of the C1Ⅱu state of acetylene. Additionally,the two components, 420 (μ1Ⅱu) and 420(к1Ⅱu),are suggested to exhibit in the present absorption spectra,due to their Renner-Teller effect and transition selection rule.All band origins and bandwidths are obtained subsequently,and it is foundthat bandwidths are broadened and lifetimes decrease gradually with the excitation of vibration.     

Control of bond-cleaving reactions of free protonated tryptophan ion by femtosecond laser pulses

The excited-state dynamics of protonated tryptophan ions is investigated by photoinduced fragmentation in the gas phase. In contrast to the neutral molecule that decays on the nanosecond time scale, the protonated species exhibits an ultrafast decay with two time constants of about 400 fs and 15 ps. In addition, after UV excitation by a pump photon at 266 nm, specific photofragments, and in particular the NH3-loss channel, can be enhanced by the absorption of a probe photon at 800 nm. The bond-cleaving reactions can thus be controlled by a variation of the pump/probe delay.     

Fourth-order Raman spectroscopy of wide-band gap materials

Low-frequency surface vibrations were observed on a rutile TiO(2)(110) surface covered with trimethyl acetate (TMA) by using fourth-order Raman spectroscopy. The TMA-covered surface interfaced to air was irradiated with 18-fs light at a wavelength of 630 nm. A pump pulse excited vibrational coherence of Raman-active modes and a probe pulse interacts with the coherently excited surface to generate second harmonic light (315 nm), the intensity of which oscillated as a function of the pump-probe delay. Four bands were recognized at 180, 357, 444, and 826 cm(-1) in the Fourier transformation spectrum of the oscillation and assigned to bulk phonons modified by the presence of the surface boundary condition. The Raman transition for the pump was nonresonant to the band gap excitation of TiO(2), as evidenced by the oscillation phase relative to the pump irradiation and by the oscillation amplitude as a function of the pump power. The observable range of this surface-selective spectroscopy is extended to wide-band gap materials on which one-photon resonance enhancement of the Raman-pump efficiency cannot be expected.     

Second harmonic generation and confined acoustic phonons in highly excited semiconductor nanocrystals

The photo-induced enhancement of second harmonic generation and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals have been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second-order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron-hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement. Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes. The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.     

Structure and property of the hydrogen bonding complex between triazines and water

The study of the intermolecular interactions that drive the solvation of six-membered nitrogenated aromatic rings is of particular importance since they are known to constitute key building blocks of pro- teins and nucleotides[1―5]. The investigation of the 1:1 adduct of these molecules with water will be the first step in the understanding of such interactions. These molecules possess two different proton-acceptor sites: the ring π cloud and the lone pairs of electrons on the nitrogen atoms…     

Relaxation and anharmonic couplings of the O-H stretching vibration of asymmetric strongly hydrogen-bonded complexes

We present infrared transient grating measurements of complexes of formic acid with pyridine and pyrazine at four excitation frequencies within the broad proton-stretching band. These experiments investigate the mechanism of the line broadening of the O-H stretching vibration. The transients show coherent oscillations that decay within a few hundred femtoseconds and population relaxation on two time scales. We fit the data using a simple model of three coupled oscillators that relax via sequential kinetics through an intermediate state. Based on this model, we conclude that the coherent oscillations result from superpositions of Fermi-resonance-coupled states involving formic acid overtone and combination states.