Picosecond Time-resolved Raman Spectroscopy Research Articles

Energy Transfer Characteristics of Lipid Bilayer Membranes of Liposomes Examined with Picosecond Time-Resolved Raman Spectroscopy.

A number of biochemical reactions proceed inside biomembranes. Since the rate of a chemical reaction is influenced by chemical properties of the surrounding environment, it is important to examine the chemical environment inside the biomembranes. Although the energy transfer characteristics are a basic and important property of a reaction medium, experimental investigation of the thermal conducting capabilities of the biomembranes is a challenging task. We have examined the energy transfer characteristics of lipid bilayer membranes of liposomes, a good model system for the biomembrane, with picosecond time-resolved Raman spectroscopy. The cooling kinetics of the first excited singlet (S1) state of trans-stilbene solubilized within the lipid bilayer membranes is observed as a peak shift of the 1570 cm-1 Raman band of S1 trans-stilbene. The cooling rate constant of S1 trans-stilbene is obtained in six lipid bilayer membranes formed by phospholipids with different hydrocarbon chains, DSPC, DPPC, DMPC, DLPC, DOPC, and egg-PC. We estimate the thermal diffusivity of the lipid bilayer membranes with a known correlation between the cooling rate constant and the thermal diffusivity of the solvent. The thermal diffusivity estimated for the liquid-crystal-phase lipid bilayer membranes is 8.9 × 10-8 to 9.4 × 10-8 m2 s-1, while that for the gel-phase lipid bilayer membranes is 8.4 × 10-8 to 8.5 × 10-8 m2 s-1. The difference in thermal diffusivity between the two phases is explained by a one-dimensional diffusion equation of heat.

Local structures in ionic liquids probed and characterized by microscopic thermal diffusion monitored with picosecond time-resolved Raman spectroscopy

Vibrational cooling rate of the first excited singlet (S(1)) state of trans-stilbene and bulk thermal diffusivity are measured for seven room temperature ionic liquids, C(2)mimTf(2)N, C(4)mimTf(2)N, C(4)mimPF(6), C(5)mimTf(2)N, C(6)mimTf(2)N, C(8)mimTf(2)N, and bmpyTf(2)N. Vibrational cooling rate measured with picosecond time-resolved Raman spectroscopy reflects solute-solvent and solvent-solvent energy transfer in a microscopic solvent environment. Thermal diffusivity measured with the transient grating method indicates macroscopic heat conduction capability. Vibrational cooling rate of S(1) trans-stilbene is known to have a good correlation with bulk thermal diffusivity in ordinary molecular liquids. In the seven ionic liquids studied, however, vibrational cooling rate shows no correlation with thermal diffusivity; the observed rates are similar (0.082 to 0.12 ps(-1) in the seven ionic liquids and 0.08 to 0.14 ps(-1) in molecular liquids) despite large differences in thermal diffusivity (5.4-7.5 × 10(-8) m(2) s(-1) in ionic liquids and 8.0-10 × 10(-8) m(2) s(-1) in molecular liquids). This finding is consistent with our working hypothesis that there are local structures characteristically formed in ionic liquids. Vibrational cooling rate is determined by energy transfer among solvent ions in a local structure, while macroscopic thermal diffusion is controlled by heat transfer over boundaries of local structures. By using "local" thermal diffusivity, we are able to simulate the vibrational cooling kinetics observed in ionic liquids with a model assuming thermal diffusion in continuous media. The lower limit of the size of local structure is estimated with vibrational cooling process observed with and without the excess energy. A quantitative discussion with a numerical simulation shows that the diameter of local structure is larger than 10 nm. If we combine this lower limit, 10 nm, with the upper limit, 100 nm, which is estimated from the transparency (no light scattering) of ionic liquids, an order of magnitude estimate of local structure is obtained as 10 nm < L < 100 nm, where L is the length or the diameter of the domain of local structure.

Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy in sooting flames

The development of time-resolved dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy using picosecond laser pulses is investigated for use in the study of highly sooting flames. Axi-symmetric ethylene and propane diffusion flames were studied. Suppression of nonresonant and Raman resonant interference signals is demonstrated by delaying the probe pulse beyond the temporal envelope of the pump pulses, and these spectra are compared with those obtained using conventional polarization-based interference suppression techniques. Flame profiles for both temperature and the O2 to N2 ratio are obtained. Evaluated temperatures are corrected for delay-induced spectral heating and compared to published thermocouple measurements.

Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy for N_2 thermometry

Time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy using picosecond-duration laser pulses is investigated for gas thermometry. The use of picosecond laser pulses significantly reduces background caused by scattering of the probe beam, and delayed probing of the Raman coherence enables elimination of interference from nonresonant four-wave mixing processes. Temperatures inferred from rotational spectra are sensitive to the probe delay because of the rotational-level dependence of collisional dephasing of Raman coherences. The sensitivity decreases, however, with increasing temperature, and accurate temperature measurements in a flame are demonstrated using a standard frequency-domain analysis of the spectra.

Vibrational Cooling Process of S1 trans-Stilbene in Ionic Liquids Observed with Picosecond Time-resolved Raman Spectroscopy

Abstract Vibrational cooling rates in room temperature ionic liquids were measured with picosecond time-resolved Raman spectroscopy. The 1570-cm−1 Raman band of the first excited singlet (S1) state of trans-stilbene was used as a “Raman thermometer.” The recorded vibrational cooling rates in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf2N) were close to those in ordinary molecular solvents despite a large difference in thermal diffusivity. The result is consistent with the assumption that local structures are formed in the ionic liquids.

Time-resolved Raman evidence for energy ‘funneling’ through propionate side chains in heme ‘cooling’ upon photolysis of carbonmonoxy myoglobin

The heme ‘cooling’ following CO photolysis was investigated with picosecond time-resolved anti-Stokes Raman spectroscopy on modified myoglobins (Mb), in which the 6- or 7-propionate is selectively replaced by a methyl group. The time constants of population decay of vibrationally excited states for two modified Mbs became significantly larger compared with those of native Mb. This is the first experimental evidence for appreciable contribution of each heme-propionate side chain to the vibrational energy transfer from the heme to the surroundings.

Kerr-gated time-resolved Raman spectroscopy of equine cortical bone tissue

Picosecond time-resolved Raman spectroscopy in equine cortical bone tissue is demonstrated. Using 400-nm pulsed laser excitation (1 ps at 1 kHz) it is shown that Kerr cell gating with a 4-ps window provides simultaneously time-resolved rejection of fluorescence and time-resolved Raman scatter enabling depth profiling through tissue. The Raman shifts are the same as those observed by conventional cw Raman spectroscopy using deep-red or near-infrared lasers. The time decay of Raman photons is shown to fit an inverse square root of time function, suggesting propagation by a diffusive mechanism. Using polystyrene behind a bone specimen, it is shown that the 400-nm laser light penetrates at least 0.31 mm below the surface of a fully mineralized bone tissue specimen and generates observable bone Raman scatter (approximately 415 to 430 nm) through most of this depth. These novel results demonstrate great promise for in vivo applications for studying diseased bone tissue, and ways to optimize the setup are discussed.

Time-resolved infrared and resonance Raman studies of benzil. Vibrational analysis and structures of the excited states

Structures of the S1 and T1 states of benzil are examined based on the experimental results from nanosecond time-resolved infrared spectroscopy and picosecond time-resolved Raman spectroscopy. Nanosecond time-resolved infrared spectra of the T1 state of benzil as well as its three isotopically substituted analogues were measured in carbon tetrachloride. The observed infrared bands of T1 benzil were assigned based on the frequency shifts on isotopic (18O, and deuteration) substitutions. The infrared band at 1312 cm−1 is assigned to the CO anti-symmetric stretch vibration. An infrared band that has large contribution from the central C–C stretch is not observed. Picosecond time-resolved resonance Raman spectra of the S1 state of benzil were also measured. It has been reported that after the photoexcitation, the benzil molecule shows an ultrafast conformational change in the S1 state. The observed resonance Raman bands are attributable to the vibrations of the relaxed form of the S1 state. By comparing the Raman and infrared spectra of the S0, S1, and T1 states of benzil, the structures of benzil in the excited states are discussed. Upon going from the S0 state to the S1 or T1 state, the bond order of the CO bond decreases while that of the central C–C bond increases. Although several ground-state bands appear in both the infrared and Raman spectra, there is no band observed simultaneously in the infrared and Raman spectra of the T1 state, except for bands attributable to the phenyl ring vibrations. We conclude that T1 benzil has the inversion center that arises from the trans-planar structure. The spectral pattern of the resonance Raman scattering of the relaxed S1 state is very similar to that of the T1 state. This implies that the molecular structure of the relaxed S1 state is similar to that of the T1 state. The structure of the relaxed form of the S1 state is also considered to be trans-planar.

Picosecond Time-Resolved Coherent Anti-Stokes Raman Spectroscopy of the Artificial Bacteriorhodopsin Pigment, BR6.11

The picosecond molecular dynamics in an artificial bacteriorhodopsin (BR) pigment, BR6.11, are measured by picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS) and picosecond transient absorption (PTA). The BR6.11 pigment contains a structurally modified retinal chromophore with a six-membered carbon ring bridging the C11C12−C13 bonds, which both locks the C11C12 bond in the trans configuration and prevents rotation about the C12−C13 bond. The changes in the vibrational degrees of freedom of the retinal attributable to the six-membered carbon ring are found in the picosecond resonance CARS (PR/CARS) spectrum of ground-state BR6.11. Normal mode assignments for more than forty BR6.11 vibrational features are made through comparisons with the PR/CARS data from native BR-570 (previously analyzed in terms of the selective isotopic substitution of the retinal). PTR/CARS spectra from two intermediates (J6.11 and K6.11), observed by PTA to appear during the initial 200 ps of the BR6.11 phot...

Picosecond Time-Resolved Raman Spectroscopy of Solids: Capabilities and Limitations for Fluorescence Rejection and the Influence of Diffuse Reflectance

It has been known for many years that it should be possible to discriminate between Raman and fluorescence phenomena on the basis of their differing temporal responses. However, it is only relatively recently that optical technology has advanced sufficiently to achieve the necessary combination of high repetition rate and picosecond laser pulses, coupled with “gateable” multichannel detectors with matched repetition rates and short on-times. Both electronic and optical gating technologies have been shown to significantly improve the Raman spectra of highly fluorescent solutions. However, the performance of such systems with solid materials has not been reported in detail. To partially redress this imbalance, this article describes the ps-time-resolved Raman spectroscopy of solid films and powders. Excellent temporal resolution and fluorescence rejection was obtained with homogeneous films, but with powders, multiple scattering has the potential to significantly blur the time resolution. For example, after incidence of a 1-ps pulse on a powdered sample of trans-stilbene, the Rayleigh signal was spread over 100 ps in time and the Raman signal persisted for more than 300 ps. Simple models are presented that predict these temporal responses on the assumption that photons randomly “diffuse” through the powder, scattering at particle boundaries and sometimes reemerging to be detected at a later time. These dynamics imply that fluorescence rejection with bulk powders might be less effective than with homogeneous solids as the broadened Raman signal would be incompletely captured within the short detector “on” period. The fluorescence would be rejected, but so would the Raman signal (to some extent), giving a poor signal-to-noise ratio. This long-term signal persistence could also complicate the interpretation of pump-probe spectroscopy studies. However, further work is needed to assess the practical implications of these findings.

Time-Resolved Raman Studies of Photoionization of Aromatic Compounds in Polar Solvents: Picosecond Relaxation Dynamics of Aromatic Cation Radicals

Picosecond time-resolved Raman spectroscopy has been used to study the ultrafast relaxation dynamics of aromatic cation radicals following two-photon ionization. In acetonitrile, integrated Raman intensities due to the cation radicals rise in tens of picoseconds, and reach their maxima at a delay time of 40−60 ps from the pump pulse. Such a slow-rise component is observed in all the cation radicals treated (biphenyl, trans-stilbene and naphthalene), suggesting that the picosecond relaxation process increasing the cation Raman intensities occurs after the photoionization of aromatic molecules. In weak polar solvents such as ethyl acetate, on the other hand, only an instrumental-limited rise (<5 ps) is observed. The rise time of the cation Raman intensity does not correlate with the dielectric relaxation time but depends on the polarity of the solvent. This result indicates that the picosecond relaxation process is not controlled by the dielectric solvent relaxation alone. The positional changes and the band narrowings of the cation Raman bands occur on a 10−20 ps time scale. These are associated with intermolecular vibrational relaxation of the cation radical toward a thermal equilibrium with solvents. The time scale of the intermolecular vibrational relaxation is the same as that of the rise component of the cation Raman intensity. From these observations, it is suggested that the thermal excitation of the solvent shell disturbs the solvation structure of the cation radical, which causes the observed picosecond change in the cation Raman intensity.

Picosecond Time-Resolved Resonance Raman Study of the Photoisomerization of Retinal

Picosecond time-resolved Raman spectroscopy was applied to the study of the photoisomerization dynamics of all-trans, 9-cis and 13-cis retinal in nonpolar solvents. It was found that picosecond time-resolved spontaneous Raman spectra are obtainable from retinal in solution despite a high fluorescence background when the probe wavelength is in rigorous resonance with the T−T absorption. In the case of photoexcitation of all-trans retinal, the transient Raman bands ascribed to the all-trans T1 state appeared with the intersystem crossing time of ∼30 ps. No Raman signal attributable to the product was recognized within the signal-to-noise ratio, reflecting the low isomerization quantum yield of all-trans retinal. The frequency shifts of the all-trans T1 bands were observed in the early picosecond time region (τ ∼ 16 ps), which manifests the vibrational cooling process in the excited state. In the case of photoexcitation of 9-cis retinal, the all-trans T1 state slowly appeared with a time constant of ∼1 ns (1000 ± 150 ps), which corresponds to the 9-cis → all-trans structural change occurring in the T1 state. In addition, Raman signals due to the 9-cis T1 state (e.g., 1400 cm-1) were recognized in the early delay time and they disappeared in accordance with the appearance of the all-trans T1 state. The data obtained clearly showed that the 9-cis → all-trans photoisomerization predominantly takes place in the T1 state with thermal activation to cross the potential barrier from the 9-cis configuration to the all-trans. In contrast, with photoexcitation of 13-cis retinal, the transient Raman signals attributable to the mixture of the all-trans T1 state and the 13-cis T1 state appeared in a few tens of picoseconds, and no spectral change was observed after 100 ps up to a few nanoseconds. The quantitative analysis indicated that the all-trans T1 state and the 13-cis T1 state appeared with different time constants. It suggests that the 13-cis → all-trans isomerization takes place in the excited singlet state before the intersystem crossing and that the resultant all-trans S1 and 13-cis S1 states are relaxed to the corresponding T1 states separately with time constants inherent to each isomer. The singlet isomerization quantum yield was estimated approximately at ∼0.2 from the obtained picosecond Raman data. These results indicated that the singlet mechanism is a major pathway (or one of major pathways) in photoisomerization of 13-cis retinal. The present time-resolved Raman study showed that the cis → trans photoisomerization mechanism and dynamics of retinal significantly depend on the position of the double bond to rotate.

Mode-specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer

Vibrational relaxation in the electronic ground state initiated by intramolecular back-electron transfer (b-ET) of betaine-30 (B-30) is studied by picosecond time-resolved anti-Stokes Raman spectroscopy. Measurements were carried out with B-30 dissolved in slowly as well as in rapidly relaxing solvents. We observed a risetime of the Raman band with the highest frequency near 1600 cm−1 which is close to the b-ET time τb-ET of B-30. For B-30 dissolved in propylene carbonate (τb-ET∼1 ps), the population of this mode exhibits a rise time of 1 ps whereas vibrational populations between 400 and 1400 cm−1 increase substantially slower. In contrast, in glycerol triacetin (τb-ET∼3.5 ps) and in ethanol (τb-ET∼6 ps) rise times of all modes are close to the respective b-ET times. Within the first few picoseconds, direct vibrational excitation through b-ET is favored for modes with the highest frequencies and high Franck–Condon factors. Later on, indirect channels of population due to vibrational energy redistribution (IVR) become effective. Thermal equilibrium populations of the Raman active modes are established within 10 to 15 ps after optical excitation.

Characterization of stimulated Raman scattering of hydrogen and methane gases as a light source for picosecond time-resolved Raman spectroscopy

Stimulated Raman scattering (SRS) in compressed hydrogen and methane gas was characterized in terms of pulse energy, temporal width and spectral width in the range of gas pressures 10‐60 atm to use it as a light source for picosecond time-resolved resonance Raman spectroscopy. SRS was pumped by the second harmonic of a Ti : sapphire oscillator‐regenerative amplifier laser system with pulse energy up to 200 mJ, duration2:5 ps and repetition rate 1 kHz. The output spectral region 421‐657 nm was covered by the first and second Stokes SRS components on tuning of the pump wavelength in the range 375‐425 nm. Energy conversion to the first Stokes SRS component was more than 10% with H 2 and more than 20% with CH4. The temporal width of the SRS pulse (1.1‐2.1 ps) was shorter than that of the pump pulse. The spectral band shape was found to be modulated, since the SRS is generated in a transient regime. When more than 100 pulses were averaged, the temporal and spectral profiles of SRS pulses were sufficiently smooth and energy fluctuations were sufficiently small for spectroscopic applications. On the basis of the results obtained, optimized conditions as a Raman shifter were deduced. The Raman shifter served as a light source for two-color pump‐probe time-resolved resonance Raman (TR 3 ) experiments and to demonstrate its capabilities, picosecond TR 3 spectra of nickel tetraphenylporphyrin in toluene solution were measured.

Picosecond Time-Resolved Raman Study of trans-Azobenzene

The electronic and vibrational relaxation of photoexcited trans-azobenzene was investigated in solution by picosecond time-resolved Raman spectroscopy. Picosecond time-resolved Raman spectra were measured with the probe wavelength at 410 nm, which is in resonance with a transient absorption appearing after the S2(ππ*) ← S0 photoexcitation. Several transient Raman bands assignable to the S1 state were observed immediately after photoexcitation. The lifetime of the S1 state showed a significant solvent dependence, and it was determined as ∼12.5 ps in ethylene glycol and ∼1 ps in hexane. Time-resolved anti-Stokes Raman measurements were also carried out for hexane solutions to obtain information about vibrational relaxation process. The anti-Stokes spectra showed that the observed S1 state was highly vibrationally excited. In addition, several anti-Stokes Raman bands due to the S0 state were observed after the decay of the S1 state, indicating that the vibrationally excited S0 state was generated after elect...

Vibrational Spectrum of a Picosecond Intermediate in the Artificial BR5.12 Photoreaction: Picosecond Time-Resolved CARS of T5.12

The vibrational spectrum of an intermediate, T5.12, in the photoreaction of an artificial bacteriorhodopsin (BR) pigment containing a five-membered carbon ring spanning the C12−C13C14 bonds (BR5.12) is measured by picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS). Observed initially by picosecond transient absorption (PTA) measurements, T5.12 is the only intermediate in the BR5.12 photoreaction (i.e., T5.12 decays only to BR5.12). BR5.12 does not have a photocycle analogous to that in native BR, presumably because the five-membered ring blocks the reaction coordinate leading to C13C14 bond isomerization. Since T5.12 may therefore represent the molecular events (reaction coordinates) that precede C13C14 bond isomerization, its vibrational spectrum may aid in elucidating the primary reaction coordinate(s) in the BR photocycle. Although T5.12 is identified via a red-shifted absorption (660 nm maximum, <3 ps formation with 3 ps BR5.12 excitation and decay in 17 ± 1 ps), no spectrosco...

Vibrational Spectrum of the J-625 Intermediate in the Room Temperature Bacteriorhodopsin Photocycle

The vibrational spectrum (800−1700 cm-1 region) of the J-625 intermediate, formed within 200−500 fs (3.5 ps decay time to K-590) in the room-temperature bacteriorhodopsin (BR) photocycle, is measured via picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS). An examination of the excitation conditions and BR photocycle kinetics, as well as the vibrational CARS data, convincingly demonstrates that these PTR/CARS spectra can be quantitatively analyzed in terms of primarily BR-570 and J-625 by using third-order nonlinear susceptibility (χ(3)) relationships. The resultant background-free (Lorentzian line shapes) CARS spectrum contains 24 distinct vibrational features which provide the most complete structural characterization of J-625 yet reported. Comparisons of the J-625 vibrational spectrum with those of ground-state BR-570 and the K-590 intermediate show that J-625 maintains some structural similarities with BR-570 while it has a significantly different structure than that of K-590. ...

Solute-Solvent Energy Transfer And Cooling Kinetics of Photoexcited S1Trans-Stilbene Probed With Picosecond Raman Spectroscopy: Comparison Between Chloroform and Chloroform-d Solutions

Cooling kinetics of trans-stilbene (tSB) photoexcited to a vibrationally excited state in the first excited singlet (S1) manifold was studied with picosecond time-resolved Raman spectroscopy. The observed cooling curves of S1tSB in chloroform (CHCI3) and in chloroform-d (CDCl3) were identical within the experimental uncertainties. For S1tSB in chloroform, there was no experimental evidence for strong intermolecular vibration-vibration (V–V) resonance energy transfer in the picosecond to sub-picosecond time region.

Reactive Intermediates in the Room Temperature Rhodopsin Photo-Reactions: Picosecond Time-Resolved Cars

The vibrational spectra of the batho and lumi intermediates in the room-temperature rhodopsin photo-reaction are measured by picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS). The instrumental principles underlying PTR/ CARS together with the advantages of PTR/CARS applications in protein reactions are discussed.

Recent Developments in Ultrafast Time-Resolved Vibrational Spectroscopy of Electronically Excited States

Developments of three new time-resolved vibrational spectroscopies and their applications to electronically excited states are reviewed. Transform-limited picosecond time-resolved Raman spectroscopy has been used to study the vibrational dynamics of trans-stilbene in the lowest excited singlet state. Picosecond time-frequency two-dimensional multiplex Coherent Antistokes Raman Scattering spectroscopy has been used to probe the structure of diphenylacetylene in the lowest and the second lowest excited singlet states. Nanosecond time-resolved dispersive infrared spectroscopy has detected the singlet and triplet intramolecular charge transfer states of 4-(dimethylamino)benzonitrile. Strong evidence for a charge transfer structure has been obtained.