Evidence for cobalt-cobalt bond homolysis and wavelength-dependent CO loss in (µ2-alkyne)Co2(CO)6 complexes.

Time-resolved infrared spectroscopy was used to probe the photochemistry of three (µ(2)-alkyne)Co(2)(CO)(6) complexes. The data indicate the formation of a triplet diradical species, with lifetimes in the range 38-71 ps. Theoretical calculations support these experimental findings. No evidence for the CO loss species, (µ(2)-alkyne)Co(2)(CO)(5), was observed, and this is rationalized by the low quantum yield for this process at the excitation wavelengths used.

A combined theoretical and experimental study on the role of spin states in the chemistry of Fe(CO)5 photoproducts.

A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragments (3)Fe(CO)(4) and (3)Fe(CO)(3) formed upon photolysis of Fe(CO)(5). Experimental data are provided for reactions in liquid n-heptane and in supercritical Xe (scXe) and Ar (scAr). Measurement of the temperature dependence of the rate of decay of (3)Fe(CO)(4) to produce (1)Fe(CO)(4)L (L = heptane or Xe) shows that these reactions have significant activation energies of 5.2 (+/-0.2) and 7.1 (+/-0.5) kcal mol(-1) respectively. Nonadiabatic transition state theory is used to predict rate constants for ligand addition, based on density functional theory calculations of singlet and triplet potential energy surfaces. On the basis of these results a new mechanism (spin-crossover followed by ligand addition) is proposed for these spin forbidden reactions that gives good agreement with the new experimental results as well as with earlier gas-phase measurements of some addition rate constants. The theoretical work accounts for the different reaction order observed in the gas phase and in some condensed phase experiments. The reaction of (3)Fe(CO)(4) with H(2) cannot be easily probed in n-heptane since conversion to (1)Fe(CO)(4)(heptane) dominates. scAr doped with H(2) provides a unique environment to monitor this reaction--Ar cannot be added to form (1)Fe(CO)(4)Ar, and H(2) addition is observed instead. Again theory accounts for the reactivity and also explains the difference between the very small activation energy measured for H(2) addition in the gas phase (Wang, W. et al. J. Am. Chem. Soc. 1996, 118, 8654) and the larger values obtained here for heptane and Xe addition in solution.

Luminescence and time-resolved infrared study of dyads containing (diimine)Ru(4,4'-diethylamido-2,2'-bipyridine)2 and (diimine)Ru(CN)4 moieties: solvent-induced reversal of the direction of photoinduced energy-transfer.

The exploitation of the dramatic negative solvatochromism of the [Ru(bipy)(CN)(4)] moiety (bipy = 2,2'-bipyridine) allows a change in solvent to reverse the direction of photoinduced energy transfer (PEnT) in two related dinuclear complexes. Both dyads consist of a [Ru(bpyam)(2)L(n)](2+) (Ru-bpyam) unit (bpyam = 4,4'-diethylamido-2,2'-bipyridine; L(n) = bis-bipyridyl-based bridging ligand) and a [Ru(L(n))(CN)(4)](2-) (Ru-CN) unit. Both termini have IR-active spectroscopic handles (amide carbonyl or cyanide, respectively) allowing the excited-state dynamics to be studied by time-resolved IR (TRIR) spectroscopy. One dyad (1) contains a relatively rigid exoditopic macrocyclic bis-bipyridyl bridging ligand (L(1)) and the other (2) contains a more flexible bis-bipyridyl bridging ligand with only one covalent linkage between the two bipyridyl binding sites (L(2)). The conformational effects on PEnT rates in these dyads are probed using a combination of luminescence and TRIR studies. In both 1 and 2 in D(2)O it is demonstrated that Ru-CN --> Ru-bpyam PEnT occurs (PEnT time scales were in the range 10 ps-3 ns) because the (3)MLCT energy of the Ru-CN terminus is higher than that of the Ru-bpyam terminus. Changing the solvent from D(2)O to CH(3)CN results in lowering the (3)MLCT energy of the Ru-CN unit below that of the Ru-bpyam unit such that in both dyads a reversal in the direction of PEnT to Ru-bpyam --> Ru-CN (time scales of 10 ps-2 ns) occurs. Complex kinetic behavior results from the presence of a dark (3)MLCT excited state formulated as {(bpyam)(2)Ru(3+)(L(n*-))} and by the presence of multiple conformers in solution which have different Ru...Ru separations giving rise to different energy transfer rates.

A time-resolved infrared vibrational spectroscopic study of the photo-dynamics of crystalline materials.

Time-resolved infrared vibrational spectroscopy is a structurally sensitive probe of the excited-state properties of matter. The technique has found many applications in the study of molecules in dilute solution phase but has rarely been applied to crystalline samples. We report on the use of a sensitive pump-probe time-resolved infrared spectrometer and sample handling techniques for studies of the ultrafast excited-state dynamics of crystalline materials. The charge transfer excited states of crystalline metal carbonyls and the proton transfer of dihydroxyquinones are presented and compared with the solution phase.

Balance between ultrafast parallel reactions in the green fluorescent protein has a structural origin.

The fluorescence photocycle of the green fluorescent protein is functionally dependent on the specific structural protein environment. A direct relationship between equilibrium protein side-chain conformation of glutamate 222 and reactivity is established, particularly the rate of ultrafast proton transfer reactions in the fluorescence photocycle. We show that parallel transformations in the photocycle have a structural origin, and we report on the vibrational properties of responsive amino acids on an ultrafast timescale. Blue excitation of GFP drives two parallel, excited-state deuteron transfer reactions with 10 ps and 75 ps time constants to the buried carboxylic acid side chain of glutamate 222 via a hydrogen-bonding network. Assignment of 1456 cm(-1) and 1441 cm(-1) modes to nu(sym) and assignment of 1564 cm(-1) and 1570 cm(-1) features to nu(asym) of E222 in the 10 ps and 75 ps components, respectively, was possible from the analysis of the transient absorption data of an E222D mutant and was consistent with photoselection measurements. In contrast to the wild-type, measurements of E222D can be described with only one difference spectrum, with the nu(sym) mode at 1435 cm(-1) and the nu(asym) mode at 1567 cm(-1), also correlating a large Deltanu(asym-sym) with slow excited-state proton transfer kinetics. Density Functional Theory calculations and published model compound and theoretical studies relate differences in Deltanu(asym-sym) to the strength and number of hydrogen-bonding interactions that are detected via equilibrium geometry and COO- stretching frequency differences of the carboxylate. The correlation of photocycle kinetics with side-chain conformation of the acceptor suggests that proton transfer from S205 to E222 controls the rate of the overall excited-state proton transfer process, which is consistent with recent theoretical predictions. Photoselection measurements show agreement for localized C=O vibrations of chromophore, Q69, and E222 with Density Functional Theory and ab initio calculations placed in the x-ray geometry and provide their vibrational response in the intermediates in the photocycle.

An alternate proton acceptor for excited-state proton transfer in green fluorescent protein: rewiring GFP.

The neutral form of the chromophore in wild-type green fluorescent protein (wtGFP) undergoes excited-state proton transfer (ESPT) upon excitation, resulting in characteristic green (508 nm) fluorescence. This ESPT reaction involves a proton relay from the phenol hydroxyl of the chromophore to the ionized side chain of E222, and results in formation of the anionic chromophore in a protein environment optimized for the neutral species (the I* state). Reorientation or replacement of E222, as occurs in the S65T and E222Q GFP mutants, disables the ESPT reaction and results in loss of green emission following excitation of the neutral chromophore. Previously, it has been shown that the introduction of a second mutation (H148D) into S65T GFP allows the recovery of green emission, implying that ESPT is again possible. A similar recovery of green fluorescence is also observed for the E222Q/H148D mutant, suggesting that D148 is the proton acceptor for the ESPT reaction in both double mutants. The mechanism of fluorescence emission following excitation of the neutral chromophore in S65T/H148D and E222Q/H148D has been explored through the use of steady state and ultrafast time-resolved fluorescence and vibrational spectroscopy. The data are contrasted with those of the single mutant S65T GFP. Time-resolved fluorescence studies indicate very rapid (< 1 ps) formation of I* in the double mutants, followed by vibrational cooling on the picosecond time scale. The time-resolved IR difference spectra are markedly different to those of wtGFP or its anionic mutants. In particular, no spectral signatures are apparent in the picosecond IR difference spectra that would correspond to alteration in the ionization state of D148, leading to the proposal that a low-barrier hydrogen bond (LBHB) is present between the phenol hydroxyl of the chromophore and the side chain of D148, with different potential energy surfaces for the ground and excited states. This model is consistent with recent high-resolution structural data in which the distance between the donor and acceptor oxygen atoms is < or = 2.4 A. Importantly, these studies indicate that the hydrogen-bond network in wtGFP can be replaced by a single residue, an observation which, when fully explored, will add to our understanding of the various requirements for proton-transfer reactions within proteins.

Ultrafast excited state dynamics of Pt(II) chromophores bearing multiple infrared absorbers.

The paper reports the synthesis, structural characterization, electrochemistry, ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopy associated with two independent d (8) square planar Pt(II) diimine chromophores, Pt(dnpebpy)Cl 2 ( 1) and Pt(dnpebpy)(C[triple bond]Cnaph) 2 ( 2), where dnpebpy = 4,4'-(CO 2CH 2- (t) Bu) 2-2,2'-bipyridine and CCnaph = naphthylacetylide. The neopentyl ester substitutions provided markedly improved complex solubility relative to the corresponding ethyl ester which facilitates synthetic elaboration as well as spectroscopic investigations. Following 400 nm pulsed laser excitation in CH 2Cl 2, the 23 cm (-1) red shift in the nu C=O vibrations in 1 are representative of a complex displaying a lowest charge-transfer-to-diimine (CT) excited state. The decay kinetics in 1 are composed of two time constants assigned to vibrational cooling of the (3)CT excited-state concomitant with its decay to the ground state (tau = 2.2 +/- 0.4 ps), and to cooling of the formed vibrationally hot ground electronic state (tau = 15.5 +/- 4.0 ps); we note that an assignment of the latter to a ligand field state cannot be excluded. Ultrafast TA data quantitatively support these assignments yielding an excited-state lifetime of 2.7 +/- 0.4 ps for the (3)CT excited-state of 1 and could not detect any longer-lived species. The primary intention of this study was to develop a Pt (II) complex ( 2) bearing dual infrared spectroscopic tags (C[triple bond]C attached to the metal and CO (ester) attached to the diimine ligand) to independently track the movement of charge density in different segments of the molecule following pulsed light excitation. Femtosecond laser excitation of 2 in CH 2Cl 2 at 400 nm simultaneously induces a red-shift in both the nu C=O (-30 cm (-1)) and the nu C[triple bond]C (-61 cm (-1)) vibrations. The TRIR data in 2 are consistent with a charge transfer assignment, and the significant decrease of the energy of the nu C[triple bond]C vibration suggests a considerable contribution from the acetylide ligands in the highest occupied molecular orbital. Therefore, we assign the lowest energy optical transitions in 2 as a combination of metal-to-ligand and ligand-to-ligand charge transfers. The excited-state of 2 is emissive at RT, with an emission maximum at 715 nm, quantum yield of 0.0012, and lifetime of 23 ns.

Excited state dynamics of a PtII diimine complex bearing a naphthalene-diimide electron acceptor.

A combination of picosecond time-resolved infrared spectroscopy, picosecond transient absorption spectroscopy, and nanosecond flash photolysis was used to elucidate the nature and dynamics of a manifold of the lowest excited states in Pt(phen-NDI)Cl 2 ( 1), where NDI = strongly electron accepting 1,4,5,8-naphthalene-diimide group. 1 is the first example of a Pt (II)-diimine-diimide dyad. UV/vis/IR spectroelectrochemistry and EPR studies of electrochemically generated anions confirmed that the lowest unoccupied molecular orbital (LUMO) in this system is localized on the NDI acceptor group. The lowest allowed electronic transition in Pt(phen-NDI)Cl 2 is charge-transfer-to-diimine of a largely Pt-->phen metal-to-ligand charge-transfer (MLCT) character. Excitation of 1 in the 355-395 nm range initiates a series of processes which involve excited states with the lifetimes of 0.9 ps ( (1)NDI*), 3 ps ( (3)MLCT), 19 ps (vibrational cooling of "hot" (3)NDI and of "hot" NDI ground state), and 520 mus ( (3)NDI). Excitation of 1 with 395 nm femtosecond laser pulses populates independently the (1)MLCT and the (1)NDI* excited states. A thermodynamically possible decay of the initially populated (1)MLCT to the charge-transfer-to-NDI excited state, [Pt (III)(phen-NDI (-*))Cl 2], is not observed. This finding could be explained by an ultrafast ISC of the (1)MLCT to the (3)MLCT state which lies about 0.4 eV lower in energy than [Pt (III)(phen-NDI (-*))Cl 2]. The predominant decay pathway of the (3)MLCT is a back electron transfer process with approximately 3 ps lifetime, which also causes partial population of the vibrationally hot ground state of the NDI fragment. The decay of the (1)NDI* state in 1 populates vibrationally hot ground state of the NDI, as well as vibrationally hot (3)NDI. The latter relaxes to form (3)NDI state, that is, [Pt(phen- (3)NDI)Cl 2]*, which possesses a remarkably long lifetime for a Pt (II) complex in fluid solution of 520 mus. The IR signature of this excited state includes the nu(CO) bands at 1607 and 1647 cm (-1), which are shifted considerably to lower energies if compared to their ground-state counterparts. The assignment of the vibrational bands is supported by the density-functional theory calculations in CH 2Cl 2. Pt(phen-NDI)Cl 2 acts as a modest photosensitizer of singlet oxygen.

Solvation-driven excited-state dynamics of [Re(4-Et-Pyridine)(CO)3(2,2'-bipyridine)]+ in imidazolium ionic liquids. A time-resolved infrared and phosphorescence study.

Excited-state dynamics of [Re(Etpy)(CO)3(bpy)]+ was studied in three imidazolium ionic liquids by time-resolved IR and emission spectroscopy on the picosecond to nanosecond time scale. Low-lying excited states were characterized by TD-DFT calculations, which also provided molecular dipole moment vectors in the relevant electronic states. TRIR spectra in ionic liquids show initial populations of two excited states: predominantly bpy-localized 3IL and 3MLCT, characterized by nu(CO) bands shifted to lower and higher frequencies, respectively, relative to the ground state. Internal conversion of 3IL to the lowest triplet 3MLCT occurred on a time scale commensurate with solvent relaxation. The nu(CO) IR bands of the 3MLCT state undergo a dynamic shift to higher wavenumbers during relaxation. Its three-exponential kinetics were determined and attributed to vibrational cooling (units of picoseconds), energy dissipation to the bulk solvent (tens of picoseconds), and solvent relaxation, the lifetime of which increases with increasing viscosity: [EMIM]BF4 (330 ps) < [BMIM]BF4 (470 ps) < [BMIM]PF6 (1570 ps). Time-resolved phosphorescence spectra in [BMIM]PF6 show a approximately 2 ns drop in intensity due to the 3IL --> 3MLCT conversion and a dynamic Stokes shift to lower energies with a lifetime decreasing from 1.8 ns at 21 degrees C to 1.1 ns at 37 degrees C, due to decreasing viscosity of the ionic liquid. It is proposed that solvent relaxation predominantly involves collective translational motions of ions. It drives the 3IL --> 3MLCT conversion, increases charge reorganization in the lowest excited-state 3MLCT, and affects vibrational anharmonic coupling, which together cause the dynamic shift of excited-state IR bands. TRIR spectroscopy of carbonyl-diimine complexes emerges as a new way to investigate various aspects of solvation dynamics, while the use of slowly relaxing ionic liquids offers new insight into the photophysics of Re(I) carbonyl polypyridyls.

The early picosecond photophysics of Ru(II) polypyridyl complexes: a tale of two timescales.

The early picosecond time scale excited-state dynamics of the paradigm tris(2,2'-bipyridyl)Ruthenium(II) ([Ru(bpy)(3)](2+)) and related complexes have been examined by picosecond Kerr-gated time-resolved resonance Raman (ps-TR(3)) spectroscopy. The evolution of the signature Raman bands of the lowest thermally equilibrated excited (THEXI) state under two-color pump/probe conditions show that this state is not fully populated within several hundred femtoseconds as proposed previously but rather only within the first 20 ps following excitation. In addition to an emission observed within the instrument rise time (τ < 3 ps), the early picosecond dynamics are characterized by a rise in the intensity of the Raman marker bands of the THEXI-(3)MLCT state, a rise time which, within experimental uncertainty, is not influenced by either partial or complete ligand deuteriation or the presence of ligands other than bpy, as in the heteroleptic complexes [Ru(bpy)(2)(L1)](+) and [Ru(bpy)(2)(Hdcb)](+) (where H(2)dcb is 4,4'-dicarboxy-2,2'-bipyridine and L1 is 2,-(5'-phenyl-4'-[1,2,4]triazole-3'-yl)pyridine). Overall, although the results obtained in the present study are consistent with those obtained from examination of this paradigm complex on the femtosecond timescale, regarding initial formation of the vibrationally hot (3)MLCT state by ISC from the singlet Franck-Condon state, the observation that the THEXI-(3)MLCT state reaches thermal equilibration over a much longer time period than previously suggested warrants a re-examination of views concerning the rapidity with which thermal equilibration of transition metal complex excited states takes place.

Ultrafast Electronic and Vibrational Dynamics of Stabilized A State Mutants of the Green Fluorescent Protein (GFP): Snipping the Proton Wire.

Two blue absorbing and emitting mutants (S65G/T203V/E222Q and S65T at pH 5.5) of the green fluorescent protein (GFP) have been investigated through ultrafast time resolved infra-red (TRIR) and fluorescence spectroscopy. In these mutants, in which the excited state proton transfer reaction observed in wild type GFP has been blocked, the photophysics are dominated by the neutral A state. It was found that the A* excited state lifetime is short, indicating that it is relatively less stabilised in the protein matrix than the anionic form. However, the lifetime of the A* state can be increased through modifications to the protein structure. The TRIR spectra show that a large shifts in protein vibrational modes on excitation of the A* state occurs in both these GFP mutants. This is ascribed to a change in H-bonding interactions between the protein matrix and the excited state.

Ultrafast structural dynamics in BLUF domains: transient infrared spectroscopy of AppA and its mutants.

The structural dynamics following photoexcitation of a photosensing BLUF (blue light sensing using FAD) domain protein have been investigated by ultrafast transient infrared spectroscopy. Specifically, the transcriptional antirepressor AppA from Rhodobacter sphaeroides has been studied in the light and dark adapted forms and in photoactive and inactive mutants W104F and Q63L. A transient absorption has been observed at 1666 cm(-1) which is a marker mode for the photoactive state of the protein. This instantaneously formed transient is tentatively assigned to a vibrational mode of a protein residue modified through its interaction with the excited state of the chromophore. A plausible candidate consistent with the mutant studies is the carbonyl stretch of the Q63 amide side chain. These results suggest that modification of the strength of protein chromophore H-bonded interactions is the primary step in the BLUF domain photocycle. No new species were observed to be formed during the first nanosecond. Measurement of the ultrafast ground state recovery showed that the excited state of light adapted AppA is strongly quenched compared to the dark adapted state. It is proposed that the reorganization which occurs to form the signaling state is favorable to electron-transfer quenching.

Picosecond infrared probing of the vibrational spectra of transients formed upon UV excitation of stacked G-tetrad structures.

The photophysical properties of stacked G-tetrads in diverse systems, including concentrated solutions of 5'-guanosine monophosphate (5'-GMP), polyguanylic acid (poly(G)) and the G-rich oligodeoxynucleotide sequence characteristic of human telomeric DNA, are probed by ps-TRIR and compared to those of the monomeric 5'-GMP.

Ultrafast IR spectroscopy of the short-lived transients formed by UV excitation of cytosine derivatives.

A strong infrared band at 1574 cm(-1) is observed following 267 nm excitation of 2'-deoxycytidine (tau = 37 +/- 4 ps) or 2'-deoxycytidine 5'-monophosphate (tau = 33 +/- 4 ps); this band is provisionally attributed to an 1n(N)pi* state and is absent for cytosine.

Ultrafast vibrational spectroscopy of the flavin chromophore.

Ultrafast time-resolved infrared (TRIR) spectra of flavin adenine dinucleotide (FAD) and the anion of lumiflavin (Lf-) are described. Ground-state recovery and excited-state decay of FAD reveal a common dominant ultrafast relaxation and a minor slower component. The Lf- transient lacks a fast component. No intermediate species are observed, suggesting that the quenching mechanism is internal conversion promoted by interaction of the adenine and isoalloxazine rings in FAD. Modes are assigned, and the potential for extension of the TRIR method to photoactive proteins is discussed.

Proton relay reaction in green fluorescent protein (GFP): Polarization-resolved ultrafast vibrational spectroscopy of isotopically edited GFP.

The complex transient vibrational spectra of wild type (wt) GFP have been assigned through polarization anisotropy measurements on isotopically edited proteins. Protein chromophore interactions modify considerably the vibrational structure, compared to the model chromophore in solution. An excited-state vibrational mode yields information on excited-state electronic structure. The proton relay pathway is characterized in more detail, and the protonation of the remote E222 residue is shown to occur in a concerted step. Modifications to protein vibrational modes are shown to occur following electronic excitation, and the potential for these to act as a trigger to the proton relay reaction is discussed.

Noninvasive Raman spectroscopy of human tissue in vivo.

We report the first transcutaneous Raman spectrum of human bone in vivo obtained at skin-safe laser illumination levels. The spectrum of thumb distal phalanx was obtained using spatially offset Raman spectroscopy (SORS), which provides chemically specific information on deep layers of human tissue, well beyond the reach of existing comparative approaches. The spectroscopy is based on collecting Raman spectra away from the point of laser illumination using concentric rings of optical fibers. As a generic analytical tool this approach paves the way for a range of uses including disease diagnosis, noninvasive probing of pharmaceutical products, biofilms, catalysts, paints, and in dermatological applications.

Structural events in the photocycle of green fluorescent protein.

Picosecond time-resolved mid-infrared absorption changes of the wild type green fluorescent protein from Aequorea victoria are reported on structural events during the photocycle. Concomitant with rapid H/D transfer following excitation of the neutral A state at 400 nm, a transient signal at 1721/1711 cm(-1) (H/D) developed belonging to protonated glutamate 222, which was definitively assigned using the E222D mutant from the altered proton-transfer kinetics to aspartate in addition to the altered band position and intensity in the spectra. A transient at 1697 cm(-1), assigned to a structural perturbation of glutamine 69, had a H/D kinetic isotope effect of >32, showing the conformational dynamics to be sensitive to the active site H/D vibrations. The kinetic data up to 2 ns after excitation in the 1250-1800 cm(-1) region in D2O provided 10 and 75 ps time constants for the excited-state deuteron transfer and the associated A1* - A1 and A2* - A2 difference spectra and showed the radiative intermediate I state vibrations and the transient accumulation of the long-lived ground-state intermediate I2. Assignments of chromophore modes for the A1, A2, and I2 ground states are proposed on the basis of published model compound studies (Esposito, A. P.; Schellenberg, P.; Parson, W. W.; Reid, P. J. J. Mol. Struct. 2001, 569, 25 and He, X.; Bell, A. F.; Tonge, P. J. J. Phys. Chem. B 2002, 106, 6056). Tentative assignments for the singlet-state intermediates A1*, A2*, and I* are discussed. An unexpected and unassigned band that may be a C=C chromophore vibration was observed in the ground state (1665 cm(-1)) as well as in all photocycle intermediates. Optical dumping of the transient I population was achieved using an additional 532 nm pulse and the directly obtained I2 - I* difference spectrum was highly similar to the I2 - I* photocycle spectrum. The pump-dump-probe spectrum differed from the pump-probe photocycle difference spectrum with respect to the intensity of the phenol 1 mode at 1578 cm(-1), suggesting stronger delocalization of the negative charge onto the phenolic ring of the anionic chromophore in the dumped I2 state. Indication for structural heterogeneity of the chromophore, Glu 222, and the chromophore environment was found in the two parallel proton-transfer reactions and their distinct associated ground- and intermediate-state vibrations. Vibrational spectral markers at 1695 cm(-1) assigned to Gln 69, at 1631 cm(-1) belonging to a C=C mode, and at 1354 cm(-1) belonging to a phenolate vibration further indicated the I2 and I* states to be unrelaxed.

Temporal and spatial resolution in transmission Raman spectroscopy.

Picosecond time-resolved transmission Raman data were acquired for 1 mm thick powder samples of trans-stilbene, and a Monte Carlo model was developed that can successfully model the laser and Raman pulse profiles. Photon migration broadened the incident (approximately 1 ps) probe pulse by two orders of magnitude. As expected from previous studies of Raman photon migration in backscattering mode, the transmitted Raman pulse was broader than the transmitted laser pulse and took longer to propagate through the sample. The late-arriving photons followed tortuous flight paths in excess of 50 mm on traversing the 1 mm sample. The Monte Carlo code was also used to study the spatial resolution (lateral and depth) of steady-state transmission Raman spectroscopy in the diffusion regime by examining the distribution of Raman generation positions as a function of incident beam size, sample thickness, and transport length. It was predicted that the lateral resolution should worsen linearly with sample thickness (typically the resolution was about 50% of the sample thickness), and this is an inevitable consequence of operating in the diffusion regime. The lateral resolution was better at the sample surface (essentially determined by the probe beam diameter or the collection aperture) than for buried objects, but transmission sampling was shown to be biased towards the mid-point of thick samples. Time-resolved transmission experiments should improve the lateral resolution by preferentially detecting snake photons, subject to constraints of signal-to-noise ratio.

Optically induced cis-trans isomerisation of (eta6-cis-stilbene)Cr(CO)3.

The femto- and picosecond time resolved spectroscopy of (eta(6)-cis-stilbene)Cr(CO)(3) provides evidence for three optically accessible excited states one leading to cis-trans isomerisation of the coordinated stilbene ligand and another which results in a slow release of CO.