Inhomogeneous Encoding of the Visual Field in the Mouse Retina.

Stimulus characteristics of the mouse's visual field differ above and below the skyline. Here, we show for the first time that retinal ganglion cells (RGCs), the output neurons of the retina, gradually change their functional properties along the ventral-dorsal axis to allow better representation of the different stimulus characteristics. We conducted two-photon targeted recordings of transient-Offα-RGCs and found that they gradually became more sustained along the ventral-dorsal axis, revealing >5-fold-longer duration responses in the dorsal retina. Using voltage-clamp recordings, pharmacology, and genetic manipulation, we demonstrated that the primary rod pathway underlies this variance. Our findings challenge the current belief that RGCs of the same subtype exhibit the same light responses, regardless of retinal location, and suggest that networks underlying RGC responses may change with retinal location to enable optimized sampling of the visual image.

[MEDICAL CANNABIS - A SOURCE FOR A NEW TREATMENT FOR AUTOIMMUNE DISEASE?].

Medical uses of Cannabis sativa have been known for over 6,000 years. Nowadays, cannabis is mostly known for its psychotropic effects and its ability to relieve pain, even though there is evidence of cannabis use for autoimmune diseases like rheumatoid arthritis centuries ago. The pharmacological therapy in autoimmune diseases is mainly based on immunosuppression of diffefent axes of the immune system while many of the drugs have major side effects. In this review we set out to examine the rule of Cannabis sativa as an immunomodulator and its potential as a new treatment option. In order to examine this subject we will focus on some major autoimmune diseases such as diabetes type I and rheumatoid arthritis.

Thermal decomposition mechanisms of alkylimidazolium ionic liquids with cyano-functionalized anions.

Because of the unusually high heats of vaporization of room-temperature ionic liquids (RTILs), volatilization of RTILs through thermal decomposition and vaporization of the decomposition products can be significant. Upon heating of cyano-functionalized anionic RTILs in vacuum, their gaseous products were detected experimentally via tunable vacuum ultraviolet photoionization mass spectrometry performed at the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. Experimental evidence for di- and trialkylimidazolium cations and cyano-functionalized anionic RTILs confirms thermal decomposition occurs primarily through two pathways: deprotonation of the cation by the anion and dealkylation of the imidazolium cation by the anion. Secondary reactions include possible cyclization of the cation and C2 substitution on the imidazolium, and their proposed reaction mechanisms are introduced here. Additional evidence supporting these mechanisms was obtained using thermal gravimetric analysis-mass spectrometry, gas chromatography-mass spectrometry, and temperature-jump infrared spectroscopy. In order to predict the overall thermal stability in these ionic liquids, the ability to accurately calculate both the basicity of the anions and their nucleophilicity in the ionic liquid is critical. Both gas phase and condensed phase (generic ionic liquid (GIL) model) density functional theory calculations support the decomposition mechanisms, and the GIL model could provide a highly accurate means to determine thermal stabilities for ionic liquids in general.

Pyrolysis of furan in a microreactor.

A silicon carbide microtubular reactor has been used to measure branching ratios in the thermal decomposition of furan, C4H4O. The pyrolysis experiments are carried out by passing a dilute mixture of furan (approximately 0.01%) entrained in a stream of helium through the heated reactor. The SiC reactor (0.66 mm i.d., 2 mm o.d., 2.5 cm long) operates with continuous flow. Experiments were performed with a reactor inlet pressure of 100-300 Torr and a wall temperature between 1200 and 1600 K; characteristic residence times in the reactor are 60-150 µs. The unimolecular decomposition pathway of furan is confirmed to be: furan (+ M) ⇌ α-carbene or ß-carbene. The α-carbene fragments to CH2=C=O + HC≡CH while the ß-carbene isomerizes to CH2=C=CHCHO. The formyl allene can isomerize to CO + CH3C≡CH or it can fragment to H + CO + HCCCH2. Tunable synchrotron radiation photoionization mass spectrometry is used to monitor the products and to measure the branching ratio of the two carbenes as well as the ratio of [HCCCH2]/[CH3C≡CH]. The results of these pyrolysis experiments demonstrate a preference for 80%-90% of furan decomposition to occur via the ß-carbene. For reactor temperatures of 1200-1400 K, no propargyl radicals are formed. As the temperature rises to 1500-1600 K, at most 10% of the decomposition of CH2=C=CHCHO produces H + CO + HCCCH2 radicals. Thermodynamic conditions in the reactor have been modeled by computational fluid dynamics and the experimental results are compared to the predictions of three furan pyrolysis mechanisms. Uncertainty in the pressure-dependency of the initiation reaction rates is a possible a source of discrepancy between experimental results and theoretical predictions.

Dissociative photoionization of glycerol and its dimer occurs predominantly via a ternary hydrogen-bridged ion-molecule complex.

The photoionization and dissociative photoionization of glycerol are studied experimentally and theoretically. Time-of-flight mass spectrometry combined with vacuum ultraviolet synchrotron radiation ranging from 8 to 15 eV is used to investigate the nature of the major fragments and their corresponding appearance energies. Deuterium (1,1,2,3,3-D5) and (13)C (2-(13)C) labeling is employed to narrow down the possible dissociation mechanisms leading to the major fragment ions (C3H(x)O2(+), C2H(x)O2(+), C2H(x)O(+), CH(x)O(+)). We find that the primary fragmentation of the glycerol radical cation (m/z 92) occurs only via two routes. The first channel proceeds via a six-membered hydrogen-transfer transition state, leading to a common stable ternary intermediate, comprised of neutral water, neutral formaldehyde, and a vinyl alcohol radical cation, which exhibits a binding energy of ≈42 kcal/mol and a very short (1.4 Å) hydrogen bond. Fragmentation of this intermediate gives rise to experimentally observed m/z 74, 62, 44, and 45. Fragments m/z 74 and 62 both consist of hydrogen-bridged ion-molecule complexes with binding energy >25 kcal/mol, whereas the m/z 44 species lacks such stabilization. This explains why water- or formaldehyde-loss products are observed first. The second primary fragmentation route arises from cleaving the elongated C-C bond. Also for this channel, intermediates comprised of hydrogen-bridged ion-molecule complexes exhibiting binding energies >24 kcal/mol are observed. Energy decomposition analysis reveals that electrostatic and charge-transfer interactions are equally important in hydrogen-bridged ion-molecule complexes. Furthermore, the dissociative photoionization of the glycerol dimer is investigated and compared to the main pathways for the monomeric species. To a first approximation, the glycerol dimer radical cation can be described as a monomeric glycerol radical cation in the presence of a spectator glycerol, thus giving rise to a dissociation pattern similar to that of the monomer.

Proton transfer in nucleobases is mediated by water.

Water plays a central role in chemistry and biology by mediating the interactions between molecules, altering energy levels of solvated species, modifying potential energy profiles along reaction coordinates, and facilitating efficient proton transport through ion channels and interfaces. This study investigates proton transfer in a model system comprising dry and microhydrated clusters of nucleobases. With mass spectrometry and tunable vacuum ultraviolet synchrotron radiation, we show that water shuts down ionization-induced proton transfer between nucleobases, which is very efficient in dry clusters. Instead, a new pathway opens up in which protonated nucleobases are generated by proton transfer from the ionized water molecule and elimination of a hydroxyl radical. Electronic structure calculations reveal that the shape of the potential energy profile along the proton transfer coordinate depends strongly on the character of the molecular orbital from which the electron is removed; i.e., the proton transfer from water to nucleobases is barrierless when an ionized state localized on water is accessed. The computed energetics of proton transfer is in excellent agreement with the experimental appearance energies. Possible adiabatic passage on the ground electronic state of the ionized system, though energetically accessible at lower energies, is not efficient. Thus, proton transfer is controlled electronically, by the character of the ionized state, rather than statistically, by simple energy considerations.

A VUV photoionization study of the multichannel reaction of phenyl radicals with 1,3-butadiene under combustion relevant conditions.

We studied the reaction of phenyl radicals (C(6)H(5)) with 1,3-butadiene (H(2)CCHCHCH(2)) exploiting a high temperature chemical reactor under combustion-like conditions (300 Torr, 873 K). The reaction products were probed in a supersonic beam by utilizing VUV radiation from the Advanced Light Source and by recording the experimental PIE curves at mass-to-charge ratios of m/z = 130 (C(10)H(10)(+)), 116 (C(9)H(8)(+)), and 104 (C(8)H(8)(+)). Our data suggest that the atomic hydrogen (H), methyl (CH(3)), and vinyl (C(2)H(3)) losses are open with estimated branching ratios of about 86 ± 4%, 8 ± 2%, and 6 ± 2%, respectively. The isomer distributions were probed further by fitting the experimentally recorded PIE curves with a linear combination of the PIE curves of individual C(10)H(10), C(9)H(8), and C(8)H(8) isomers. These fits indicate the formation of three C(10)H(10) isomers (trans-1,3-butadienylbenzene, 1,4-dihydronaphthalene, 1-methylindene), three C(9)H(8) isomers (indene, phenylallene, 1-phenyl-1-methylacetylene), and a C(8)H(8) isomer (styrene). A comparison with results from recent crossed molecular beam studies of the 1,3-butadiene-phenyl radical reaction and electronic structure calculations suggests that trans-1,3-butadienylbenzene (130 amu), 1,4-dihydronaphthalene (130 amu), and styrene (104 amu) are reaction products formed as a consequence of a bimolecular reaction between the phenyl radical and 1,3-butadiene. 1-Methylindene (130 amu), indene (116 amu), phenylallene (116 amu), and 1-phenyl-1-methylacetylene (116 amu) are synthesized upon reaction of the phenyl radical with three C(4)H(6) isomers: 1,2-butadiene (H(2)CCCH(CH(3))), 1-butyne (HCCC(2)H(5)), and 2-butyne (CH(3)CCCH(3)); these C(4)H(6) isomers can be formed from 1,3-butadiene via hydrogen atom assisted isomerization reactions or via thermal rearrangements of 1,3-butadiene involving hydrogen shifts in the high temperature chemical reactor.

Molecular beam mass spectrometry with tunable vacuum ultraviolet (VUV) synchrotron radiation.

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy and dynamics(1-4). Fundamental studies of photoionization processes of biomolecules provide information about the electronic structure of these systems. Furthermore determinations of ionization energies and other properties of biomolecules in the gas phase are not trivial, and these experiments provide a platform to generate these data. We have developed a thermal vaporization technique coupled with supersonic molecular beams that provides a gentle way to transport these species into the gas phase. Judicious combination of source gas and temperature allows for formation of dimers and higher clusters of the DNA bases. The focus of this particular work is on the effects of non-covalent interactions, i.e., hydrogen bonding, stacking, and electrostatic interactions, on the ionization energies and proton transfer of individual biomolecules, their complexes and upon micro-hydration by water(1, 5-9). We have performed experimental and theoretical characterization of the photoionization dynamics of gas-phase uracil and 1,3-dimethyluracil dimers using molecular beams coupled with synchrotron radiation at the Chemical Dynamics Beamline(10) located at the Advanced Light Source and the experimental details are visualized here. This allowed us to observe the proton transfer in 1,3-dimethyluracil dimers, a system with pi stacking geometry and with no hydrogen bonds(1). Molecular beams provide a very convenient and efficient way to isolate the sample of interest from environmental perturbations which in return allows accurate comparison with electronic structure calculations(11, 12). By tuning the photon energy from the synchrotron, a photoionization efficiency (PIE) curve can be plotted which informs us about the cationic electronic states. These values can then be compared to theoretical models and calculations and in turn, explain in detail the electronic structure and dynamics of the investigated species (1, 3).

Thermal decomposition of CH3CHO studied by matrix infrared spectroscopy and photoionization mass spectroscopy.

A heated SiC microtubular reactor has been used to decompose acetaldehyde and its isotopomers (CH(3)CDO, CD(3)CHO, and CD(3)CDO). The pyrolysis experiments are carried out by passing a dilute mixture of acetaldehyde (roughly 0.1%-1%) entrained in a stream of a buffer gas (either He or Ar) through a heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 50-200 Torr with the SiC tube wall temperature in the range 1200-1900 K. Characteristic residence times in the reactor are 50-200 µs after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 µTorr. The reactor has been modified so that both pulsed and continuous modes can be studied, and results from both flow regimes are presented. Using various detection methods (Fourier transform infrared spectroscopy and both fixed wavelength and tunable synchrotron radiation photoionization mass spectrometry), a number of products formed at early pyrolysis times (roughly 100-200 µs) are identified: H, H(2), CH(3), CO, CH(2)=CHOH, HC≡CH, H(2)O, and CH(2)=C=O; trace quantities of other species are also observed in some of the experiments. Pyrolysis of rare isotopomers of acetaldehyde produces characteristic isotopic signatures in the reaction products, which offers insight into reaction mechanisms that occur in the reactor. In particular, while the principal unimolecular processes appear to be radical decomposition CH(3)CHO (+M) → CH(3) + H + CO and isomerization of acetaldehyde to vinyl alcohol, it appears that the CH(2)CO and HCCH are formed (perhaps exclusively) by bimolecular reactions, especially those involving hydrogen atom attacks.

Ionization of dimethyluracil dimers leads to facile proton transfer in the absence of hydrogen bonds.

Proton transfer is ubiquitous in chemistry and biology, occurring, for example, in proteins, enzyme reactions and across proton channels and pumps. However, it has always been described in the context of hydrogen-bonding networks ('proton wires') acting as proton conduits. Here, we report efficient intramolecular ionization-induced proton transfer across a 1,3-dimethyluracil dimer, a model π-stacked system with no hydrogen bonds. Upon photoionization by tunable vacuum ultraviolet synchrotron radiation, the dimethyluracil dimer undergoes proton transfer and dissociates to produce a protonated monomer. Deuterated dimethyluracil experiments confirm that proton transfer occurs from the methyl groups and not from the aromatic C-H sites. Calculations reveal qualitative differences between the proton transfer reaction coordinate in the π-stacked and hydrogen-bonded base pairs, and that proton transfer in methylated dimers involves significant rearrangements of the two fragments, facilitating a relatively low potential energy barrier of only 0.6 eV in the ionized dimer.

A VUV photoionization study of the combustion-relevant reaction of the phenyl radical (C6H5) with propylene (C3H6) in a high temperature chemical reactor.

We studied the reaction of phenyl radicals (C(6)H(5)) with propylene (C(3)H(6)) exploiting a high temperature chemical reactor under combustion-like conditions (300 Torr, 1200-1500 K). The reaction products were probed in a supersonic beam by utilizing tunable vacuum ultraviolet (VUV) radiation from the Advanced Light Source and recording the photoionization efficiency (PIE) curves at mass-to-charge ratios of m/z = 118 (C(9)H(10)(+)) and m/z = 104 (C(8)H(8)(+)). Our results suggest that the methyl and atomic hydrogen losses are the two major reaction pathways with branching ratios of 86 ± 10% and 14 ± 10%. The isomer distributions were probed by fitting the recorded PIE curves with a linear combination of the PIE curves of the individual C(9)H(10) and C(8)H(8) isomers. Styrene (C(6)H(5)C(2)H(3)) was found to be the exclusive product contributing to m/z = 104 (C(8)H(8)(+)), whereas 3-phenylpropene, cis-1-phenylpropene, and 2-phenylpropene with branching ratios of 96 ± 4%, 3 ± 3%, and 1 ± 1% could account for the signal at m/z = 118 (C(9)H(10)(+)). Although searched for carefully, no evidence of the bicyclic indane molecule could be provided. The reaction mechanisms and branching ratios are explained in terms of electronic structure calculations nicely agreeing with a recent crossed molecular beam study on this system.

Thermal decomposition mechanism of 1-ethyl-3-methylimidazolium bromide ionic liquid.

In order to better understand the volatilization process for ionic liquids, the vapor evolved from heating the ionic liquid 1-ethyl-3-methylimidazolium bromide (EMIM(+)Br(-)) was analyzed via tunable vacuum ultraviolet photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS) and thermogravimetric analysis mass spectrometry (TGA-MS). For this ionic liquid, the experimental results indicate that vaporization takes place via the evolution of alkyl bromides and alkylimidazoles, presumably through alkyl abstraction via an S(N)2 type mechanism, and that vaporization of intact ion pairs or the formation of carbenes is negligible. Activation enthalpies for the formation of the methyl and ethyl bromides were evaluated experimentally, ΔH(‡)(CH(3)Br) = 116.1 ± 6.6 kJ/mol and ΔH(‡)(CH(3)CH(2)Br) = 122.9 ± 7.2 kJ/mol, and the results are found to be in agreement with calculated values for the S(N)2 reactions. Comparisons of product photoionization efficiency (PIE) curves with literature data are in good agreement, and ab initio thermodynamics calculations are presented as further evidence for the proposed thermal decomposition mechanism. Estimates for the enthalpy of vaporization of EMIM(+)Br(-) and, by comparison, 1-butyl-3-methylimidazolium bromide (BMIM(+)Br(-)) from molecular dynamics calculations and their gas phase enthalpies of formation obtained by G4 calculations yield estimates for the ionic liquids' enthalpies of formation in the liquid phase: ΔH(vap)(298 K) (EMIM(+)Br(-)) = 168 ± 20 kJ/mol, ΔH(f, gas)(298 K) (EMIM(+)Br(-)) = 38.4 ± 10 kJ/mol, ΔH(f, liq)(298 K) (EMIM(+)Br(-)) = -130 ± 22 kJ/mol, ΔH(f, gas)(298 K) (BMIM(+)Br(-)) = -5.6 ± 10 kJ/mol, and ΔH(f, liq)(298 K) (BMIM(+)Br(-)) = -180 ± 20 kJ/mol.

Ionization of Water Clusters Mediated by Exciton Energy Transfer from Argon Clusters.

The exciton energy deposited in an argon cluster (Arn, ⟨n = 20⟩) using VUV radiation is transferred to softly ionize doped water clusters ((H2O)n, n = 1-9), leading to the formation of nonfragmented clusters. Following the initial excitation, electronic energy is channeled to ionize the doped water cluster while evaporating the Ar shell, allowing identification of fragmented and complete water cluster ions. Examination of the photoionization efficiency curve shows that cluster evaporation from excitons located above 12.6 eV is not enough to cool the energized water cluster ion and leads to their dissociation to (H2O)n-2H(+) (protonated) clusters.

A VUV Photoionization and Ab Initio Determination of the Ionization Energy of a Gas-Phase Sugar (Deoxyribose).

The ionization energy of gas-phase deoxyribose was determined using tunable vacuum ultraviolet synchrotron radiation coupled to an effusive thermal source. Adiabatic and vertical ionization energies of the ground and first four excited states of α-pyranose, the structure that dominates in the gas phase, were calculated using high-level electronic structure methods. An appearance energy of 9.1(±0.05) eV was recorded, which agrees reasonably well with a theoretical value of 8.8 eV for the adiabatic ionization energy. A clear picture of the dissociative photoionization dynamics of deoxyribose emerges from the fragmentation pattern recorded using mass spectrometry and from ab initio molecular dynamics calculations. The experimental threshold 9.4 (±0.05) eV for neutral water elimination upon ionization is captured well in the calculations, and qualitative insights are provided by molecular orbital analysis and molecular dynamics snapshots along the reaction coordinate.

The products of the thermal decomposition of CH3CHO.

We have used a heated 2 cm × 1 mm SiC microtubular (µtubular) reactor to decompose acetaldehyde: CH(3)CHO + Δ â†’ products. Thermal decomposition is followed at pressures of 75-150 Torr and at temperatures up to 1675 K, conditions that correspond to residence times of roughly 50-100 µs in the µtubular reactor. The acetaldehyde decomposition products are identified by two independent techniques: vacuum ultraviolet photoionization mass spectroscopy (PIMS) and infrared (IR) absorption spectroscopy after isolation in a cryogenic matrix. Besides CH(3)CHO, we have studied three isotopologues, CH(3)CDO, CD(3)CHO, and CD(3)CDO. We have identified the thermal decomposition products CH(3) (PIMS), CO (IR, PIMS), H (PIMS), H(2) (PIMS), CH(2)CO (IR, PIMS), CH(2)=CHOH (IR, PIMS), H(2)O (IR, PIMS), and HC≡CH (IR, PIMS). Plausible evidence has been found to support the idea that there are at least three different thermal decomposition pathways for CH(3)CHO; namely, radical decomposition: CH(3)CHO + Δ â†’ CH(3) + [HCO] → CH(3) + H + CO; elimination: CH(3)CHO + Δ â†’ H(2) + CH(2)=C=O; isomerization∕elimination: CH(3)CHO + Δ â†’ [CH(2)=CH-OH] → HC≡CH + H(2)O. An interesting result is that both PIMS and IR spectroscopy show compelling evidence for the participation of vinylidene, CH(2)=C:, as an intermediate in the decomposition of vinyl alcohol: CH(2)=CH-OH + Δ â†’ [CH(2)=C:] + H(2)O → HC≡CH + H(2)O.

Soft ionization of thermally evaporated hypergolic ionic liquid aerosols.

Isolated ion pairs of a conventional ionic liquid, 1-Ethyl-3-Methyl-Imidazolium Bis(trifluoromethylsulfonyl)imide ([Emim(+)][Tf(2)N(-)]), and a reactive hypergolic ionic liquid, 1-Butyl-3-Methyl-Imidazolium Dicyanamide ([Bmim(+)][Dca(-)]), are generated by vaporizing ionic liquid submicrometer aerosol particles for the first time; the vaporized species are investigated by dissociative ionization with tunable vacuum ultraviolet (VUV) light, exhibiting clear intact cations, Emim(+) and Bmim(+), presumably originating from intact ion pairs. Mass spectra of ion pair vapor from an effusive source of the hypergolic ionic liquid show substantial reactive decomposition due to the internal energy of the molecules emanating from the source. Photoionization efficiency curves in the near threshold ionization region of isolated ion pairs of [Emim(+)][Tf(2)N(-)] ionic liquid vapor are compared for an aerosol source and an effusive source, revealing changes in the appearance energy due to the amount of internal energy in the ion pairs. The aerosol source has a shift to higher threshold energy (∼0.3 eV), attributed to reduced internal energy of the isolated ion pairs. The method of ionic liquid submicrometer aerosol particle vaporization, for reactive ionic liquids such as hypergolic species, is a convenient, thermally "cooler" source of isolated intact ion pairs in the gas phase compared to effusive sources.

Vacuum-ultraviolet photoionization and mass spectrometric characterization of lignin monomers coniferyl and sinapyl alcohols.

The fragmentation mechanisms of monolignols under various energetic processes are studied with jet-cooled thermal desorption molecular beam (TDMB) mass spectrometry (MS), 25 keV Bi(3)(+) secondary ion MS (SIMS), synchrotron vacuum-ultraviolet secondary neutral MS (VUV-SNMS) and theoretical methods. Experimental and calculated appearance energies of fragments observed in TDMB MS indicate that the coniferyl alcohol photoionization mass spectra contain the molecular parent and several dissociative photoionization products. Similar results obtained for sinapyl alcohol are also discussed briefly. Ionization energies of 7.60 eV ± 0.05 eV for coniferyl alcohol and <7.4 eV for both sinapyl and dihydrosinapyl alcohols are determined. The positive ion SIMS spectrum of coniferyl alcohol shares few characteristic peaks (m/z = 137 and 151) with the TDMB mass spectra, shows extensive fragmentation, and does not exhibit clear molecular parent signals. VUV-SNMS spectra, on the other hand, are dominated by the parent ion and main fragments also present in the TDMB spectra. Molecular fragmentation in VUV-SNMS spectra can be reduced by increasing the extraction delay time. Some features resembling the SIMS spectra are also observed in the desorbed neutral products. The monolignol VUV-SNMS peaks shared with the TDMB mass spectra suggest that dissociative photoionization of ion-sputtered neutral molecules predominate in the VUV-SNMS mass spectra, despite the extra internal energy imparted in the initial ion impact. The potential applications of these results to imaging MS of biomolecules are discussed.

Raman spectral signatures as conformational probes of gas phase flexible molecules.

A novel application of ionization-loss stimulated Raman spectroscopy (ILSRS) for monitoring the spectral features of four conformers of a gas phase flexible molecule is reported. The Raman spectral signatures of four conformers of 2-phenylethylamine are well matched by the results of density functional theory calculations, showing bands uniquely identifying the structures. The measurement of spectral signatures by ILSRS in an extended spectral range, with a conventional laser source, is instrumental in facilitating the unraveling of intra- and intermolecular interactions that are significant in biological structure and activity.

A new method for determining absorption cross sections out of initially excited vibrational states.

A first experimental demonstration, combining the methods of vibrationally mediated photodissociation (VMP) and ionization-loss stimulated Raman spectroscopy (ILSRS) for measuring cross sections for dissociation of vibrationally excited levels is reported. The action spectrum obtained in the VMP of methylamine exhibits enhancement of the H photofragment yield as a result of initial vibrational excitation and the ILSRS monitors the fraction of molecules being excited. The partial cross sections for H production out of the sampled vibrational states and the extent of mode selectivity were thus determined.

Mode-dependent enhancement of photodissociation and photoionization in a seven atom molecule.

We report the first experimental demonstration of vibrational mode-dependent enhancement in photodissociation and photoionization of a seven atom molecule, methylamine (CH(3)NH(2)). The fundamental C-H stretches and the overtones or combinations of CH(3) bends were prepared via stimulated Raman excitation (SRE) prior to their 243.135 nm one-photon dissociation or two-photon ionization. The photodissociation or photoionization of the vibrationally excited molecules was achieved via 10 ns delayed or temporally overlapping SRE and UV pulses, respectively. It is shown that bending modes are more effective than stretches in promoting photodissociation and photoionization, since their UV excitation is favored by larger Franck Condon factors. This behavior provides clear evidence for vibrational mode-dependence in a relatively large molecule with a torsional degree of freedom, indicating that these modes survive intramolecular vibrational redistribution on a time scale considerably longer than hitherto inferred from previous studies.