Conformations of 2-aminoindan in a supersonic jet: the role of intramolecular N-H...pi hydrogen bonding

Laser-induced fluorescence (LIF), dispersed fluorescence (DF), mass-resolved one-color resonance enhanced two-photon ionization (RE2PI) and UV-UV hole-burning spectra of 2-aminoindan (2-AI) were measured in a supersonic jet. The hole-burning spectra demonstrated that the congested vibronic structures observed in the LIF excitation spectrum were responsible for three conformers of 2-AI. The origins of the conformers were observed at 36931, 36934, and 36955 cm(-1). The DF spectra obtained by exciting the band origins of the three conformers showed quite similar vibrational structures, with the exception of the bands around 600-900 cm(-1). The molecular structures of the three conformers were assigned with the aid of ab initio calculations at the MP2/6-311+G(d,p) level. An amino hydrogen of the most stable conformer points toward the benzene ring. The stability of the most stable conformer was attributed to an intramolecular N-H...pi hydrogen bonding between the hydrogen atom and the pi-electron of the benzene ring. The other two conformers, devoid of intramolecular hydrogen bonding, were also identified for 2-AI. This suggests weak hydrogen bonding in the most stable conformer. The intramolecular N-H...pi hydrogen bonding in 2-AI was discussed in comparison with other weak hydrogen-bonding systems.     

Jet spectroscopy of arylmethyl radicals in the visible region: assignment of low-frequency vibrational modes in diphenylmethyl and chlorodiphenylmethyl radicals

The D(1)-D(0) transitions of diphenylmethyl (DPM) and chlorodiphenylmethyl (CDPM) radicals were studied by laser induced fluorescence (LIF) spectroscopy in a supersonic jet. Laser induced fluorescence excitation and dispersed fluorescence (DF) spectra were obtained for DPM and CDPM radicals produced by ArF excimer laser (193 nm) photolyses of their chlorides. With the aid of the density functional theory (DFT) calculation, vibronic bands are assigned by comparing the observed LIF excitation spectra of the jet-cooled radicals with the single vibronic level DF spectra. Low-frequency vibrations of 55 and 53 cm(-1) in the ground and excited states, respectively, are assigned to the symmetric phenyl torsional mode of the DPM radical. The geometries of DPM in the ground and excited states are discussed with regards to observed spectra and DFT calculations. Similarly for the CDPM radical, symmetric phenyl torsional and Ph-C-Ph bending modes are assigned and the halogen-substitution effect in equilibrium geometry is discussed.     

Satellites of Xe transitions induced by infrared active vibrational modes of CF4 and C2F6 molecules

Absorption and luminescence excitation spectra of Xe/CF(4) mixtures were studied in the vacuum UV region at high resolution using tunable synchrotron radiation. Pressure-broadened resonance bands and bands associated with dipole-forbidden states of the Xe atom due to collision-induced breakdown of the optical selection rules are reported. The spectra display in addition numerous satellite bands corresponding to transitions to vibrationally excited states of a Xe-CF(4) collisional complex. These satellites are located at energies of Xe atom transition increased by one quantum energy in the IR active v(3) vibrational mode of CF(4) (v(3) = 1281 cm(-1)). Satellites of both resonance and dipole-forbidden transitions were observed. Satellites of low lying resonance states are spectrally broad bands closely resembling in shape their parent pressure-broadened resonance bands. In contrast, satellites of dipole-forbidden states and of high lying resonance states are spectrally narrow bands (FWHM ～10 cm(-1)). The satellites of dipole-forbidden states are orders of magnitude stronger than transitions to their parent states due to collision-induced breakdown of the optical selection rules. These satellites are attributed to a coupling of dipole-forbidden and resonance states induced by the electric field of the transient CF(4) (v(3) = 0 ? v(3) = 1) dipole. Similar satellites are present in spectra of Xe/C(2)F(6) mixtures where these bands are induced by the IR active v(10) mode of C(2)F(6). Transitions to vibrationally excited states of Xe-CF(4)(C(2)F(6)) collision pairs were also observed in two-photon LIF spectra.     

Laser spectroscopy and dynamics of the jet-cooled AsH2 free radical

The A (2)A(1)-X (2)B(1) electronic transition of the jet-cooled AsH(2) free radical has been studied by laser-induced fluorescence (LIF), wavelength-resolved emission, and fluorescence lifetime measurements. The radical was produced by a pulsed electric discharge through a mixture of arsine (AsH(3)) and high pressure argon at the exit of a pulsed valve. Nine vibronic bands were identified by LIF spectroscopy in the 505-400 nm region, including a long progression in the bending mode and two bands (1(0) (1) and 1(0) (1)2(0) (1)) involving the excited state As-H symmetric stretch. Single vibronic level emission spectra showed similar activity in the bending and symmetric stretching frequencies of the ground state. High-resolution spectra of the 0(0) (0) band exhibited large spin splittings and small, resolved arsenic hyperfine splittings, due to a substantial Fermi contact interaction in the excited state. The rotational constants obtained in the analysis gave effective molecular structures of r"(0)=1.5183(1) A, theta"(0)=90.75(1) degrees and r'(0)=1.4830(1) A, theta'(0)=123.10(2) degrees . The excited state fluorescence lifetimes vary dramatically with rovibronic state, from a single value of 1.4 micros to many with lifetimes less than 10 ns, behavior which the authors interpret as signaling the onset of a predissociative process near the zero-point level of the ground state.     

Excitation and emission spectra of rubidium in rare-gas thin-films

To understand the optical properties of atoms in solid state matrices, the absorption, excitation, and emission spectra of rubidium doped thin-films of argon, krypton, and xenon were investigated in detail. A two-dimensional spectral analysis extends earlier reports on the excitation and emission properties of rubidium in rare-gas hosts. We found that the doped crystals of krypton and xenon exhibit a simple absorption-emission relation, whereas rubidium in argon showed more complicated spectral structures. Our sample preparation employed in the present work yielded different results for the Ar crystal, but our peak positions were consistent with the prediction based on the linear extrapolation of Xe and Kr data. We also observed a bleaching behavior in rubidium excitation spectra, which suggests a population transfer from one to another spectral feature due to hole-burning. The observed optical response implies that rubidium in rare-gas thin-films is detectable with extremely high sensitivity, possibly down to a single atom level, in low concentration samples.     

Intermolecular vibrations and dynamics of the anisole—benzene complex studied by multi-resonance spectroscopy

The intermolecular vibrations of the anisole—benzene complex in the ground and excited electronic states have been observed by the LIF (laser-induced fluorescence) and fluorescence-dip techniques. Short progressions due to the intermolecular vibrations suggest a small structure change of the complex upon electronic excitation. The LIF excitation spectrum shows predominant progressions of 27 cm⁻¹, which is tentatively assigned to one of the intermolecular bending modes in the excited electronic state. On the other hand, the fluorescence-dip spectrum shows only a series of bands with irregular intervals due to the intermolecular modes in the ground electronic state. The decay rates of the vibrationally excited complex in the ground electronic state have also been measured with the SEP-LIF (stimulated emission pumping-laser-induced fluorescence) technique, where the complex vibrationally excited by SEP is probed by the delayed LIF measurements. The complex excited to its purely intermolecular mode stays in the initially prepared state after a delay time of 1 μs. On the other hand, the complex excited to the intramolecular vibrational states above 500 cm⁻¹ does not seem to stay in the prepared states. Neither the relaxed complex nor the dissociated monomer was detected. A possible reason for this observation is discussed.     

Conformational effects on vibronic spectra and excited state dynamics of 3-fluorobenzoic acid dimer

Two conformational isomers of 3-fluorobenzoic acid dimer (3-FBA(2)) have been identified in a supersonic jet expansion by use of laser-induced fluorescence excitation (FE), UV-UV hole-burning, and dispersed fluorescence (DF) spectroscopic methods. In the FE spectrum, the S(1) origins of the two isomeric species appear at a frequency gap of only 24 cm(-1), and the vibronic intensities of the redshifted dimer (dimer I) are about two times weaker than those of dimer II. However, ab initio quantum chemistry calculations at the MP2/6-31G(**) level of theory predict that all the isomeric species of 3-FBA(2) have almost the same binding energy (approximately 17 kcal/mol) in the ground state. Furthermore, unlike benzoic acid dimer, the present system shows intense activity for a low-frequency mode in both the FE and DF spectra. With the aid of DFT (B3LYP/6-311G(**)) predicted normal mode frequencies, we have assigned the mode to the in-plane gear (cogwheel) vibration of the cyclic hydrogen-bonded frame of the dimer. The Franck-Condon profiles for vibronic excitation of the mode indicate that the distortion of the cyclic hydrogen bond frame as a result of S(1)<--S(0) excitation is larger for dimer I than dimer II. Moreover, the fluorescence lifetime at the S(1) zero-point level of the former is also significantly smaller than the latter. Using the predictions of configuration interaction singles calculations, we have proposed that the spectral and dynamical differences between the two isomeric species observed in this study are manifestations of the different characteristics of their S(1) surfaces. By measuring FE, DF, and hole-burning spectra of a mixed dimer between 3-fluobenzoic acid and benzoic acid we have shown that the isomeric features in the homodimer spectra are due to two locally excited rotamers of the 3-fluorobenzoic acid moiety.     

Remote Raman and fluorescence studies of mineral samples

In the present study, we investigated remote laser-induced fluorescence (LIF), at a distance of 4.8 m, of a variety of natural minerals and rocks, and Hawaiian Ti (Cordyline terminalis) plant leaves. These minerals included calcite cleavage, calcite onex and calcite travertine, gypsum, fluorapatite, Dover flint and chalk, chalcedony and nephelene syenite, and rubies containing rock. Pulsed laser excitation of the samples at 355 and 266 nm often resulted in strong fluorescence. The LIF bands in the violet-blue region at approximately 413 and approximately 437 nm were observed only in the spectrum of calcite cleavage. The green LIF bands with band maxima in the narrow range of approximately 501-504 nm were observed in the spectra of all the minerals with the exception of the nephelene syenite and ruby rocks. The LIF red bands were observed in the range approximately 685-711 nm in all samples. Excitation with 532 nm wavelength laser gave broad but relatively low fluorescence background in the low-frequency region of the Raman spectra of these minerals. One microsecond signal gating was effective in removing nearly all background fluorescence (with peak at approximately 610 nm) from calcite cleavage Raman spectra, indicating that the fluorescence was probably from long-lifetime inorganic phosphorescence.     

Direct on-strip analysis of size- and time-resolved aerosol impactor samples using laser induced fluorescence spectra excited at 263 and 351 nm

We report a novel atmospheric aerosol characterization technique, in which dual wavelength UV laser induced fluorescence (LIF) spectrometry marries an eight-stage rotating drum impactor (RDI), namely UV-LIF-RDI, to achieve size- and time-resolved analysis of aerosol particles on-strip. The UV-LIF-RDI technique measured LIF spectra via direct laser beam illumination onto the particles that were impacted on a RDI strip with a spatial resolution of 1.2 mm, equivalent to an averaged time resolution in the aerosol sampling of 3.6 h. Excited by a 263 nm or 351 nm laser, more than 2000 LIF spectra within a 3-week aerosol collection time period were obtained from the eight individual RDI strips that collected particles in eight different sizes ranging from 0.09 to 10 μm in Djibouti. Based on the known fluorescence database from atmospheric aerosols in the US, the LIF spectra obtained from the Djibouti aerosol samples were found to be dominated by fluorescence clusters 2, 5, and 8 (peaked at 330, 370, and 475 nm) when excited at 263 nm and by fluorescence clusters 1, 2, 5, and 6 (peaked at 390 and 460 nm) when excited at 351 nm. Size- and time-dependent variations of the fluorescence spectra revealed some size and time evolution behavior of organic and biological aerosols from the atmosphere in Djibouti. Moreover, this analytical technique could locate the possible sources and chemical compositions contributing to these fluorescence clusters. Advantages, limitations, and future developments of this new aerosol analysis technique are also discussed.     

LIF spectroscopy of jet-cooled 1:1 complex between 7-azaindole and 2-pyrrolidinone

Laser-induced fluorescence (LIF) spectra of a 1:1 complex between 7-azaindole (7AI) and five-member cyclic amide 2-pyrrolidinone (2-PDN) have been measured in a supersonic free jet expansion. The bands in the excitation spectrum appear doublet, which has been attributed to splitting of the zero-point level in the ground state due to puckering of 2-PDN moiety of the complex in a symmetric double minimum potential. This feature is consistent with low puckering barrier (～260 cm^?1) predicted by electronic structure calculation. The complex emits only UV fluorescence from locally excited state in the jet, but visible tautomer fluorescence is observed in hydrocarbon solution.     

Hot bands in jet-cooled and ambient temperature spectra of chloromethylene

Rotationally resolved spectra of several bands lying to the red of the origin of the A(1)A" - X (1)A' band system of chloromethylene (HCCl), were recorded by laser absorption spectroscopy in ambient temperature and jet-cooled samples. The radical was made by excimer laser photolysis of dibromochloromethane, diluted in inert gas, at 193 nm. The jet-cooled sample showed efficient rotational but less vibrational cooling. Analysis showed that the observed bands originate in the (upsilon(1),upsilon(2),upsilon(3)) = (010), (001), and (011) vibrational levels of the ground electronic state of the radical, while the upper-state levels involved were (000), (010), (001), and (011). Vibrational energies and rotational constants describing the rotational levels in the lower-state vibrational levels were determined by fitting to combination differences. The analysis also resulted in a reevaluation of the C-Cl stretching frequency in the excited state and we find E(001)' = 13 206.57 or 926.17 cm(-1) above the A(1)A" (000) rotationless level for HC(35)Cl. Scaled ab initio potential energy surfaces for the A and X states were used to compute the transition moment surface and thereby the relative intensities of different vibronic transitions, providing additional support for the assignments and permitting the prediction of the shorter wavelength spectrum. All the observed upper state levels showed some degree of perturbation in their rotational energy levels, particularly in K(a) = 1, presumably due to coupling with near-resonant vibrationally excited levels of the ground electronic state. Transitions originating in the low-lying a(3)A" were also predicted to occur in the same wavelength region, but could not be identified in the spectra.     

Methyl torsional potentials of rotational isomers of m-methylanisole studied by spectral hole-burning spectroscopy

Laser-induced fluorescence (LIF) excitation spectra of m-methylanisole in a supersonic jet were measured. Two series of progressions were observed in the spectrum, originating at 36048 and 36115 cm⁻¹, which were successfully assigned to the transitions to the methyl internal rotational vibronic levels of the two isomers, i.e. cis and trans isomers, with the aid of hole-burning spectrum measurements and quantum-chemical calculations. The progression for the trans isomer was observed up to the 6a₁ band, while only the 3a₁ band in addition to the 0a₁ and 1e bands was observed for the cis isomer. This finding can be explained by the conformational change upon the electronic excitation; the 60° rotation of the methyl torsional angle takes place for the trans isomer but not for the cis isomer.     

Photoelectron valence band spectra of diacetylene polymers

The valence band photoelectron spectra of three single crystal diacetylene polymers excited with Mg Kα and He radiation are reported. Spectral bands due to the polymer chain are identified by elimination of side group contributions using existing data for related chemical groups and by observations of one diacetylene during polymerization. Core electron levels are used as binding energy references. The results are compared with other experimental data and with molecular orbital calculations for polyenes and polydiacetylenes.     

Dependencies of the Raman spectra of p-oligophenyls on the chain length and the excitation wavelength

The 1064 nm excited Raman spectra of p-terphenyl, p-quaterphenyl, p-quinquephenyl and p-sexiphenyl have been observed and compared with the 514.5 nm excited spectra. Dependencies of intensities of some major bands on the chain length and the excitation wavelength are discussed in terms of the preresonant Raman effect. A method for estimating the chain length from relative intensities is proposed.     

Ab initio and experimental study of the K-shell spectra of 2,5-dihydrofuran

The K-shell spectra of gaseous 2,5-dihydrofuran at the carbon and oxygen thresholds are reported for the first time. They have been measured using the inner-shell electron energy loss spectroscopy (ISEELS) method. Ab initio Configuration Interaction calculations have been carried out to assign the observed bands. The O1s spectrum is similar to that of tetrahydrofuran and the assignments of the bands are close to those obtained in the case of furan, excepting the furan first π* band. At the C1s edge, the spectrum differs from the furan case, because of the different chemical environment of one of the non-equivalent carbon atoms: due to the presence of hydrogen atoms out of the carbon–oxygen ring plane, several Rydberg core excited states have an important valence character, leading to large intensities in the experimental spectrum.     

Far-infrared spectra of some β-hydroquinone clathrates

Far-infrared spectra (400-30 cm^?1) of Nujol mulls of the β-hydroquinone clathrates containing the following guest molecules were investigated: formic acid, formic acid-d₂, methanol, methanol-d₄, acetonitrile, acetonitrile-d₃, sulphur dioxide and also both of methanol and sulphur dioxide. The observed infrared bands of the mulls in the region of 4000-30 cm^?1 were classified into those due to the host lattice and those due to the guest molecules. On the basis of the comparison of the spectra, some bands were assigned to the translational or the rotational vibrations of guest molecules. Appearance of those bands suggested that some guest molecules are considerably bound in the cavities of the host lattice. Effect of temperature change on the bands was also measured.     

Infrared and Raman spectra of N-acetyl-l-amino acid methylamides with aromatic side groups

Infrared and Raman spectra of N-acetyl-l-phenylalanine methylamide, N-acetyl-l-tyrosine methylamide and N-acetyl-l-tryptophan methylamide, as model compounds of aromatic amino acid residues in proteins, were measured in the solid state and in methanol solutions. Vibrational assignments of the spectra were made by utilizing the deuteration effect and by comparison with the spectra of related compounds which include toluene, p-cresol and 3-methylindole. The amide I, III and IV bands were strong in Raman scattering, but other characteristic amide bands were ill-defined. In the Raman spectra of methanol solutions, only the bands due to the aromatic side group vibrations were markedly observed, but those due to the peptide backbone vibrations were very weak, suggesting the coexistence of various molecular conformations in solution.     

High-spin alkali trimers on helium nanodroplets: spectral separation and analysis

Electronic excitation spectra of homo- (K(3),Rb(3)) and heteronuclear (K(2)Rb,KRb(2)) alkali trimers in the high-spin quartet state have been investigated in a broad spectral range (10,600-17,400 cm(-1)). Ten new bands showing laser induced fluorescence (LIF) were measured. Due to the pickup statistics, overlapping spectra of all possible oligomers are present at once, complicating the unraveling and assignment of individual spectra. To circumvent the problem, two variations of beam depletion spectroscopy were employed in addition to the conventional analysis of the relation between signal and pickup pressure: A two-laser V-type double resonance scheme combining beam depletion with LIF, and a mass selective beam depletion scheme. In principle, these allow accurate separation of an arbitrary number of overlapping spectra. The benefits and drawbacks of each method are discussed. Assignment to electronic states is achieved by comparison with ab initio complete active space self-consistent field calculations of the excited electronic level structure of the molecules.     

Ultraviolet absorption and vibrational spectra of 2-fluoro-5-bromopyridine

The ultraviolet absorption spectrum in the range 340-185 nm in the vapour and solution phase has been measured for 2-fluoro-5-bromopyridine. Three fairly intense band systems identified as the pi* <-- pi transitions II, III and IV have been observed. A detailed vibronic analysis of the vapor and solution spectra is presented. The first system of bands is resolved into about sixty-two distinct vibronic bands in the vapour-phase spectrum. The 0,0 band is located at 35944 cm(-1). Two well-developed progressions, in which the excited state frequencies nu'25 (283 cm(-1)) and nu'19 (550 cm(-1)) are excited by several quanta, have been observed. The corresponding excited state vibrational and anharmonicity constants are found to be omega'i = 292 cm(-1), x'ii = 4.5 cm(-1) (i = 25) and omega'i = 563.8 cm(-1), x'ii = 6.9 cm(-1) (i = 19). The other two band systems show no vibronic structure, the band maxima being located at 48346 and 52701 cm(-1), respectively. The oscillator strength of the band systems in different solutions and the excited state dipole moments associated with the first two transitions have been determined by the solvent-shift method. The infrared spectrum in the region 4000-130 cm(-1) and the laser Raman spectrum of the molecule in the liquid state have been measured and a complete vibrational assignment of the observed frequencies is given. A correlation of the ground and excited state fundamental frequencies observed in the UV absorption spectrum with the Raman or infrared frequencies is presented.