Excited-state relaxation in PbSe quantum dots

In solids the phonon-assisted, nonradiative decay from high-energy electronic excited states to low-energy electronic excited states is picosecond fast. It was hoped that electron and hole relaxation could be slowed down in quantum dots, due to the unavailability of phonons energy matched to the large energy-level spacings ("phonon-bottleneck"). However, excited-state relaxation was observed to be rather fast (< or =1 ps) in InP, CdSe, and ZnO dots, and explained by an efficient Auger mechanism, whereby the excess energy of electrons is nonradiatively transferred to holes, which can then rapidly decay by phonon emission, by virtue of the densely spaced valence-band levels. The recent emergence of PbSe as a novel quantum-dot material has rekindled the hope for a slow down of excited-state relaxation because hole relaxation was deemed to be ineffective on account of the widely spaced hole levels. The assumption of sparse hole energy levels in PbSe was based on an effective-mass argument based on the light effective mass of the hole. Surprisingly, fast intraband relaxation times of 1-7 ps were observed in PbSe quantum dots and have been considered contradictory with the Auger cooling mechanism because of the assumed sparsity of the hole energy levels. Our pseudopotential calculations, however, do not support the scenario of sparse hole levels in PbSe: Because of the existence of three valence-band maxima in the bulk PbSe band structure, hole energy levels are densely spaced, in contradiction with simple effective-mass models. The remaining question is whether the Auger decay channel is sufficiently fast to account for the fast intraband relaxation. Using the atomistic pseudopotential wave functions of Pb(2046)Se(2117) and Pb(260)Se(249) quantum dots, we explicitly calculated the electron-hole Coulomb integrals and the P-->S electron Auger relaxation rate. We find that the Auger mechanism can explain the experimentally observed P-->S intraband decay time scale without the need to invoke any exotic relaxation mechanisms.     

Time-resolved relaxation dynamics of Hgn- (11 <or = n <or = 16,n = 18) clusters following intraband excitation at 1.5 eV

Electron-nuclear relaxation dynamics are studied in Hg(n) (-) (11 相似文献   

Time-resolved intraband electronic relaxation dynamics of Hg(n) (-) clusters (n=7-13,15,18) excited at 1.0 eV

Time-resolved photoelectron imaging has been used to study the relaxation dynamics of small Hg(n) (-) clusters (n=7-13,15,18) following intraband electronic excitation at 1250 nm (1.0 eV). This study furthers our previous investigation of single electron, intraband relaxation dynamics in Hg(n) (-) clusters at 790 nm by exploring the dynamics of smaller clusters (n=7-10), as well as those of larger clusters (n=11-13,15,18) at a lower excitation energy. We measure relaxation time scales of 2-9 ps, two to three times faster than seen previously after 790 nm excitation of Hg(n) (-), n=11-18. These results, along with size-dependent trends in the absorption cross-section and photoelectron angular distribution anisotropy, suggest significant evolution of the cluster anion electronic structure in the size range studied here. Furthermore, the smallest clusters studied here exhibit 35-45 cm(-1) oscillations in pump-probe signal at earliest temporal delays that are interpreted as early coherent nuclear motion on the excited potential energy surfaces of these clusters. Evidence for evaporation of one or two Hg atoms is seen on a time scale of tens of picoseconds.     

Electron and hole injection in PbSe quantum dot films

Electrochemical methods are used to inject charge into films of colloidal semiconductor nanocrystals of PbSe. The injection of electrons and holes into quantum confined states is confirmed by monitoring changes in the IR absorption spectrum. Holes are injected into the 1Sh state, causing a bleach of the 1Sh-1Se and 1Sh-1Pe interband transitions and inducing a 1Sh-1Ph intraband absorption. Electrons can be sequentially injected into the 1Se and 1Pe states, first bleaching the 1Sh-1Se and 1Ph-1Se interband transitions and inducing a 1Se-1Pe intraband absorption, and then bleaching the 1Sh-1Pe transition and inducing a 1Pe-1De intraband absorption.     

Comprehensive investigation of vibrational relaxation of non-hydrogen-bonded water molecules in liquids

The vibrational dynamics of isolated water molecules dissolved in the nonpolar organic liquids 1,2-dichloroethane (C(2)H(4)Cl(2)) and d-chloroform (CDCl(3)) have been studied using an IR pump-probe experiment with approximately 2 ps time resolution. Analyzing transient, time, and spectrally resolved data in both the OH bending and the OH stretching region, the anharmonic constants of the bending overtone (v=2) and the bend-stretch combination modes were obtained. Based on this knowledge, the relaxation pathways of single water molecules were disentangled comprehensively, proving that the vibrational energy of H(2)O molecules is relaxing following the scheme OH stretch-->OH bend overtone-->OH bend-->ground state. A lifetime of 4.8+/-0.4 ps is determined for the OH bending mode of H(2)O in 1,2-dichloroethane. For H(2)O in CDCl(3) a numerical analysis based on rate equations suggests a bending overtone lifetime of tau(020)=13+/-5 ps. The work also shows that full 2-dimensional (pump-probe) spectral resolution with access to all vibrational modes of a molecule is required for the comprehensive analysis of vibrational energy relaxation in liquids.     

Excited-state proton transfer: indication of three steps in the dissociation and recombination process

A femtosecond pump-probe, with approximately 150 fs resolution, as well as time-correlated single photon counting with approximately 10 ps resolution techniques are used to probe the excited-state intermolecular proton transfer from HPTS to water. The pump-probe signal consists of two ultrafast components (approximately 0.8 and 3 ps) that precede the relatively slow (approximately 100 ps) component. From a comparative study of the excited acid properties in water and methanol and of its conjugate base in basic solution of water, we propose a modified mechanism for the ESPT consisting of two reactive steps followed by a diffusive step. In the first, fast, step the photoacid dissociates at about 10 ps to form a contact ion pair RO-*...H3O+. The contact ion pair recombines efficiently to re-form the photoacid with a recombination rate constant twice as large as the dissociation rate constant. The first-step equilibrium constant value is about 0.5 and thus, at short times, <10 ps, only approximately 30% of the excited photoacid molecules are in the form of the conjugated base-proton contact ion pair. In the second, slower, step, of about 100 ps, the proton is separated by at least one water molecule from the conjugate base RO-. The separated proton and the conjugated base can recombine geminately as described by our previous diffusion-assisted model. The new two-step reactive model predicts that the population of the ROH form of HPTS will decrease with two time constants and the RO- population will increase by the same time constants. The proposed model fits the experimental data of this study as well as previous published experimental data.     

Ultrafast photoinduced electron transfer in directly linked porphyrin-ferrocene dyads

The ultrafast electron transfer occurring upon Soret excitation of three new porphyrin-ferrocene (XP-Fc) dyads has been studied by femtosecond up-conversion and pump-probe techniques. In the XP-Fc dyads (XP-Fcs) designed in this study, the ferrocene moiety is covalently bonded to the meso positions of 3,5-di-tert-butylphenyl zinc porphyrin (BPZnP-Fc), pentafluorophenyl zinc porphyrin (FPZnP-Fc), and 3,5-di-tert-butylphenyl free-base porphyrin (BPH2P-Fc). Charge separation and recombination in the XP-Fcs were confirmed by transient absorption spectra, and the lifetimes of the charge-separated states were estimated from the decay rate of the porphyrin radical anion band to be approximately 20 ps. The charge-separation rates of the XP-Fcs were found to be >10(13) s-1 from the S2 state and 6.3x10(12) s-1 from the S1 state. Charge separation from the S2 state was particularly efficient for BPZnP-Fc, whereas the main reaction pathway was from the S1 state for BPH2P-Fc. Charge separation from the S2 and S1 states occurred at virtually the same rate in benzene and tetrahydrofuran and was much faster than their solvation times. Analysis of these results using semiquantum Marcus theory indicates that the magnitude of the electronic-tunneling matrix element is rather large and far outside the range of nonadiabatic approximation. The pump-probe data show the presence of vibrational coherence during the reactions, suggesting that wavepacket dynamics on the adiabatic potential energy surface might regulate the ultrafast reactions.     

Dynamics of photoinduced processes in adenine and thymine base pairs

The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited pi pi* state to an n pi* state (lifetime tau(pi pi*) approximately 100 fs) followed by a slower decay of the latter with tau(n pi*) approximately 0.9 ps for (adenine)2, tau(n pi*) = 6-9 ps for (thymine)2, and tau(n pi*) approximately 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the pi pi* state via the pi sigma* state greatly suppressed the n pi* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.     

Dynamics and thermodynamics of water in PAMAM dendrimers at subnanosecond time scales

Atomistic molecular dynamics simulations are used to study generation 5 polyamidoamine (PAMAM) dendrimers immersed in a bath of water. We interpret the results in terms of three classes of water: buried water well inside of the dendrimer surface, surface water associated with the dendrimer-water interface, and bulk water well outside of the dendrimer. We studied the dynamic and thermodynamic properties of the water at three pH values: high pH with none of the primary or tertiary amines protonated, intermediate pH with only the primary amines protonated, and low pH with all amines protonated. For all pH values we find that both buried and surface water exhibit two relaxation times: a fast relaxation ( approximately 1 ps) corresponding to the libration motion of the water and a slow ( approximately 20 ps) diffusional component related to the escaping of water from one domain to another. In contrast for bulk water the fast relaxation is approximately 0.4 ps while the slow relaxation is approximately 14 ps. These results are similar to those found in biological systems, where the fast relaxation is found to be approximately 1 ps while the slow relaxation ranges from 20 to 1000 ps. We used the 2PT MD method to extract the vibrational (power) spectrum and found substantial differences for the three classes of water. The translational diffusion coefficient for buried water is 11-33% (depending on pH) of the bulk value while the surface water is about 80%. The change in rotational diffusion is quite similar: 21-45% of the bulk value for buried water and 80% for surface water. This shows that translational and rotational dynamics of water are affected by the PAMAM-water interactions as well as due to the confinement in the interior of the dendrimer. We find that the reduction of translational or rotational diffusion is accompanied by a blue shift of the corresponding libration motions ( approximately 10 cm(-1) for translation, approximately 35 cm(-1) for rotation), indicating higher local force constants for these motions. These effects are most pronounced for the lowest pH, probably because of the increased rigidity caused by the internal charges. From the vibrational density of states we also calculate the enthalpies and entropies of the various waters. We find that water molecules are enthalpically favored near the PAMAM dendrimer: energy for surface water is approximately 0.1 kcal/mol lower to that in the bulk, and approximately 0.5-0.9 kcal/mol lower for buried water. In contrast, we find that both the buried and surface water are entropically unfavored: buried water is 0.9-2.2 kcal/mol lower than the bulk while the surface water is 0.1-0.2 kcal/mol lower. The net result is a thermodynamically unfavored state of the water surrounding the PAMAM dendrimer: 0.4-1.3 kcal/mol higher for buried water and 0.1-0.2 kcal/mol for surface water. This excess free energy of the surface and buried waters is released when the PAMAM dendrimer binds to DNA or metal ions, providing an extra driving force.     

Excited-state absorption and ultrafast relaxation dynamics of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication

The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrin dication (TOxP2+) has been investigated in the femtosecond-nanosecond time domain upon photoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kinetic profiles corresponding with the band maxima due to excited-state absorption have been determined for the three species. Four components of the relaxation process are distinguished for H2P: the unresolvably short B --> Qy internal conversion is followed by the Qy --> Qx process, vibrational relaxation, and thermalization in the Qx state with time constant approximately 150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP2+, two processes are resolved, i.e., B --> Q internal conversion and thermal equilibration in the Q state. The B --> Q time constant has been determined to be 25 ps. The large difference with respect to the B --> Qy time constant of H2P has been related to the increased energy gap between the coupled states, 9370 cm-1 in TOxP2+ vs 6100 cm-1 in H2P. The relaxation dynamics of H4P2+ has a first ultrafast component of approximately 300 fs assigned as internal conversion between the B (or Soret) state and charge-transfer (CT) states of the H4P2+ complex with two trifluoroacetate counterions. This process is followed by internal CT --> Q conversion (time constant 9 ps) and thermalization in the Q state (time constant 22 ps).     

Multidimensional infrared spectroscopy of the N-H bond motions in formamide

The heterodyned two-dimensional (2D) IR spectra and equilibrium dynamics of the N-H stretching motion of DCONHD in deuterated formamide, DCOND(2), were studied with 80 fs pulses at 3 microm. The time evolution of the heterodyned 2D IR spectra, pump-probe spectra, and photon echo peak shift demonstrate that interstate dynamics is occurring by relaxation of the original N-H excitation. The N-H vibrational frequency correlation function can be expressed as a sum of three exponentials with correlation times 0.24 ps, 0.8 ps, and 11 ps. The intermediate component is attributed to motions of the N-Hcdots, three dots, centeredO unit involving only slight angular variations of the N-H bond. The slow component is attributed to the structure breaking and making. The anisotropy decay confirmed that the significant angular N-H bond motion occurs on the 11 ps time scale. The fast component, which is the least well determined, might correspond to the modulation of the H-bond distance without angular motion. The correlation coefficient between the pumped and relaxed state distributions was +0.51, implying that the excited state phase memory is only slightly diminished by the relaxation of the N-H excitation. The relaxed modes are concluded to be local to the driven N-H mode.     

Ultrafast pump-probe study of the primary photoreaction process in pharaonis halorhodopsin: halide ion dependence and isomerization dynamics

Halorhodopsin is a retinal protein that acts as a light-driven chloride pump in the Haloarchaeal cell membrane. A chloride ion is bound near the retinal chromophore, and light-induced all- trans --> 13- cis isomerization triggers the unidirectional chloride ion pump. We investigated the primary ultrafast dynamics of Natronomonas pharaonis halorhodopsin that contains Cl (-), Br (-), or I (-) ( pHR-Cl (-), pHR-Br (-), or pHR-I (-)) using ultrafast pump-probe spectroscopy with approximately 30 fs time resolution. All of the temporal behaviors of the S n <-- S 1 absorption, ground-state bleaching, K intermediate (13- cis form) absorption, and stimulated emission were observed. In agreement with previous reports, the primary process exhibited three dynamics. The first dynamics corresponds to the population branching process from the Franck-Condon (FC) region to the reactive (S 1 (r)) and nonreactive (S 1 (nr)) S 1 states. With the improved time resolution, it was revealed that the time constant of this branching process (tau 1) is as short as 50 fs. The second dynamics was the isomerization process of the S 1 (r) state to generate the ground-state 13- cis form, and the time constant (tau 2) exhibited significant halide ion dependence (1.4, 1.6, and 2.2 ps for pHR-Cl (-), pHR-Br (-), and pHR-I (-), respectively). The relative quantum yield of the isomerization, which was evaluated from the pump-probe signal after 20 ps, also showed halide ion dependence (1.00, 1.14, and 1.35 for pHR-Cl (-), pHR-Br (-), and pHR-I (-), respectively). It was revealed that the halide ion that accelerates isomerization dynamics provides the lower isomerization yield. This finding suggests that there is an activation barrier along the isomerization coordinate on the S 1 potential energy surface, meaning that the three-state model, which is now accepted for bacteriorhodopsin, is more relevant than the two-state model for the isomerization process of halorhodopsin. We concluded that, with the three-state model, the isomerization rate is controlled by the height of the activation barrier on the S 1 potential energy surface while the overall isomerization yield is determined by the branching ratios at the FC region and the conical intersection. The third dynamics attributable to the internal conversion of the S 1 (nr) state also showed notable halide ion dependence (tau 3 = 4.5, 4.6, and 6.3 ps for pHR-Cl (-), pHR-Br (-), and pHR-I (-)). This suggests that some geometrical change may be involved in the relaxation process of the S 1 (nr) state.     

OH-stretch vibrational relaxation of HOD in liquid to supercritical D2O

The population relaxation of the OH-stretching vibration of HOD diluted in D2O is studied by time-resolved infrared (IR) pump-probe spectroscopy for temperatures of up to 700 K in the density range 12 1 OH stretching transition with a 200 fs laser pulse centered at approximately 3500 cm(-1). Above 400 K these spectra show no indication of spectral diffusion after pump-probe delays of 0.3 ps. Over nearly the entire density range and for sufficiently high temperatures (T > 360 K), the vibrational relaxation rate constant, kr, is strictly proportional to the dielectric constant, epsilon, of water. Together with existing molecular dynamics simulations, this result suggests a simple linear dependence of kr on the number of hydrogen-bonded D2O molecules. It is shown that, for a given temperature, an isolated binary collision model is able to adequately describe the density dependence of vibrational energy relaxation even in hydrogen-bonded fluids. However, dynamic hydrogen bond breakage and formation is a source of spectral diffusion and affects the nature of the measured kr. For sufficiently high temperatures when spectral diffusion is much faster than energy transfer, the experimentally observed decays correspond to ensemble averaged population relaxation rates. In contrast, when spectral diffusion and vibrational relaxation occur on similar time scales, as is the case for ambient conditions, deviations from the linear kr(epsilon) relation occur because the long time decay of the v = 1 population is biased to slower relaxing HOD molecules that are only weakly connected to the hydrogen bond network.     

Ultrafast dynamics of metal complexes of tetrasulphonated phthalocyanines

A promising material in medicine, electronics, optoelectronics, electrochemistry, catalysis, and photophysics, tetrasulphonated aluminum phthalocyanine (AlPcS(4)), is investigated by means of steady-state and time-resolved pump-probe spectroscopies. Absorption and steady-state fluorescence spectroscopy indicate that AlPcS(4) is essentially monomeric. Spectrally resolved pump-probe data are recorded on time scales ranging from femtoseconds to nanoseconds. The nature of these fast processes and pathways of the competing relaxation processes from the initially excited electronic states in aqueous and organic (dimethyl sulfoxide) solutions are discussed. The decays and bleaching recovery have been fitted in the ultrafast window (0-10 ps) and later time window extending to nanoseconds (0-1 ns). While the excited-state dynamics have been found to be sensitive to the solvent environment, we were able to show that the fast dynamics is described by three time constants in the ranges of 115-500 fs, 2-25 ps, and 150-500 ps. We were able to ascribe these three time constants to different processes. The shortest time constants have been assigned to vibrational wavepacket dynamics. The few picosecond components have been assigned to vibrational relaxation in the excited electronic states. Finally, the 150-500 ps components represent the decay from S(1) to the ground state. The experimental and theoretical treatment proposed in this paper provides a basis for a substantial revision of the commonly accepted interpretation of the Soret transition (B transition) that exists in the literature.     

Ab initio nonadiabatic dynamics study of ultrafast radiationless decay over conical intersections illustrated on the Na3F cluster

We present a theoretical approach for the ultrafast nonadiabatic dynamics based on the ab initio molecular dynamics carried out "on the fly" in the framework of the configuration interaction method combined with Tully's surface hopping algorithm for nonadiabatic transitions. This approach combined with our Wigner distribution approach allows us to perform accurate simulations of femtosecond pump-probe spectra in the systems where radiationless transitions among electronic states take place. In this paper we illustrate this by theoretical simulation of ultrafast processes and nonradiative relaxation in the Na(3)F cluster, involving three excited states and the ground electronic state. Furthermore, we show that our accurate simulation of the photoionization pump-probe spectrum is in full agreement with the experimental signal. Based on the nonadiabatic dynamics at high level of accuracy and taking into account all degrees of freedom, the nonradiative lifetime for the 1 (1)B(1) excited state of Na(3)F has been determined to be approximately 900 fs.     

Sequential merocyanine product isomerization following femtosecond UV excitation of a spiropyran

The ring-opening dynamics of the photochromic switch 1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-(2H)-indole] in tetrachloroethene is studied with both femtosecond time-resolved ultraviolet (UV)/visible and UV/mid-infrared (IR) pump-probe spectroscopy. During the first picosecond we identify two new transient features in the UV/vis experiments, the first of which we assign to spiropyran S1 --> S(n) absorption (lifetime < or = 0.2 ps). The second feature (lifetime 0.5 +/- 0.2 ps) we tentatively assign to the merocyanine T2 state. After 1 ps both probing methods show biexponential merocyanine formation kinetics, with average time constants of 17 +/- 3 and 350 +/- 20 ps. In the UV/IR experiments, the initial dynamics show more dispersion in formation times than in the UV/vis measurements, whereas the slower time constant is the same in both. A weak transient IR signal at approximately 1360 cm(-1) demonstrates that this biexponentiality is caused by a sequential isomerization between two merocyanine species. Lifetimes provide evidence that the merocyanine S1 state is not involved in the photochemical reaction.     

Rotational energy transfer in NO (A 2Sigma+,v'=0) by N2 and O2 at room temperature

State-to-state rotational energy transfer (RET) rate coefficients for NO (A 2Sigma+, v'=0, J=5.5, 11.5, 17.5) were measured for N2 and O2 at room temperature using a pump-probe method. The NO A 2Sigma+ state is prepared by 226 nm light and the RET is monitored by fluorescence from the D 2Sigma+ v'=0 state, following excitation by a time-delayed laser at approximately 1.1 microm. Additionally, total collisional removal and final state distributions were measured exciting in the Q1+P21 band head, to simulate an NO laser-induced fluorescence atmospheric monitoring scheme. Time-resolved modeling is used to understand relaxation mechanisms and predict relaxation times in ambient air. H2O at atmospherically relevant concentrations does not affect the degree of RET in ambient air.     

Ultrafast formation of the benzoic acid triplet upon ultraviolet photolysis and its sequential photodissociation in solution

Time-resolved infrared (TR-IR) absorption spectroscopy in both the femtosecond and nanosecond time domain has been applied to examine the photolysis of benzoic acid in acetonitrile solution following either 267 nm or 193 nm excitation. By combining the ultrafast and nanosecond TR-IR measurements, both the excited states and the photofragments have been detected and key mechanistic insights were obtained. We show that the solvent interaction modifies the excited state relaxation pathways and thus the population dynamics, leading to different photolysis behavior in solution from that observed in the gas phase. Vibrational energy transfer to solvents dissipates excitation energy efficiently, suppressing the photodissociation and depopulating the excited S(2) or S(3) state molecules to the lowest T(1) state with a rate of ～2.5 ps after a delayed onset of ～3.7 ps. Photolysis of benzoic acid using 267 nm excitation is dominated by the formation of the T(1) excited state and no photofragments could be detected. The results from TR-IR experiments using higher energy of 193 nm indicate that photodissociation proceeds more rapidly than the vibrational energy transfer to solvents and C-C bond fission becomes the dominant relaxation pathway in these experiments as featured by the prominent observation of the COOH photofragments and negligible yield of the T(1) excited state. The measured ultrafast formation of T(1) excited state supports the existence of the surface intersections of S(2)/S(1), S(2)/T(2), and S(1)/T(1)/T(2), and the large T(1) quantum yield of ～0.65 indicates the importance of the excited state depopulation to triplet manifold as the key factor affecting the photophysical and photochemical behavior of the monomeric benzoic acid.     

Nonmonotonic energy harvesting efficiency in biased exciton chains

We theoretically study the efficiency of energy harvesting in linear exciton chains with an energy bias, where the initial excitation is taking place at the high-energy end of the chain and the energy is harvested (trapped) at the other end. The efficiency is characterized by means of the average time for the exciton to be trapped after the initial excitation. The exciton transport is treated as the intraband energy relaxation over the states obtained by numerically diagonalizing the Frenkel Hamiltonian that corresponds to the biased chain. The relevant intraband scattering rates are obtained from a linear exciton-phonon interaction. Numerical solution of the Pauli master equation that describes the relaxation and trapping processes reveals a complicated interplay of factors that determine the overall harvesting efficiency. Specifically, if the trapping step is slower than or comparable to the intraband relaxation, this efficiency shows a nonmonotonic dependence on the bias: it first increases when introducing a bias, reaches a maximum at an optimal bias value, and then decreases again because of dynamic (Bloch) localization of the exciton states. Effects of on-site (diagonal) disorder, leading to Anderson localization, are addressed as well.     

Early Time Solvation Dynamics Probed by Spectrally Resolved Degenerate Pump-Probe Spectroscopy

The dynamic role of solvent in influencing the rates of physico-chemical processes (for example, polar solvation and electron transfer) has been extensively studied using time-resolved fluorescence spectroscopy. Here we study ultrafast excited state relaxation dynamics of three different fluorescent probes (DNTTCI, IR-140 and IR-144) in two polar solvents, ethanol and ethylene glycol, using spectrally resolved degenerate pump-probe spectroscopy. We discuss how time-resolved emission spectra can be directly used for constructing relaxation correlation function, obviating spectral reconstruction and estimation of time-zero spectrum in non-polar solvents. We show that depending on the specific probe used, the relaxation dynamics is governed either by intramolecular vibrational relaxation (for IR140) or by intermolecular solvation (for DNTTCI) or by both (for IR144). We further show (using DNTTCI as a probe) that major differences in solvation by ethanol and ethylene glycol is contributed by early time (<1 ps) dynamics.