Laser-induced Fluorescence Excitation Spectra Research Articles

The Mg–NC stretching vibronic bands of the MgNC [formula omitted] transition

We have generated MgNC in supersonic free jet expansions, and measured the laser induced fluorescence excitation spectra of the Mg–NC stretching vibronic bands of the A ˜ 2 Π – X ˜ 2 Σ + transition. In addition to the bands reported previously, the 3 0 2 and 3 0 3 bands were found, and the vibronic levels, (0, 0, 2) and (0, 0, 3), in the à 2Π state were characterized through rotational analysis. Sorting out the spin–orbit constants, A SO ′ , of the vibronic levels with respect to the two stretching modes, Mg–NC and C–N, we found the opposite dependence of A SO ′ upon the two vibrational modes, i.e., A SO ′ increases with the excitation of the Mg–NC stretching vibrational mode, while it decreases with the excitation of the C–N mode. These findings were interpreted by an attempt to calculate the dependence of A SO ′ upon the geometry along the vibrational normal modes.

Electronic spectra of molecules with two C3 v internal rotors: Torsional analysis of the [formula omitted] LIF spectrum of biacetyl

The laser-induced fluorescence excitation spectrum of the biacetyl A 1 A u ( S 1)– X 1 A g ( S 0) transition in the region from 22 182 to 22 800 cm −1 shows a complicated absorption line spectrum, which is believed to arise from a long progression in the torsional vibrations of the two equivalent methyl tops in this molecule. In this paper, we discuss three topics: (i) a numerical calculation of these energy levels using a kinetic and potential energy formalism and constants from the literature [M.L. Senent, D.C. Moule, Y.G. Smeyers, A. Toro-Labbé, F.J. Peqalver, J. Mol. Spectrosc. 164 (1994) 66], (ii) a qualitative description of the calculated energy level pattern using local-mode ideas applied to the two equivalent methyl rotors together with G 36 permutation-inversion group symmetry species, and (iii) a least-squares refinement of the torsional potential parameters based on results of some of our high-resolution rotational analyses, followed by a comparison of energy levels calculated from the refined parameters with a low-resolution spectrum taken in the region from 0 to 500 cm −1 above the A– X band origin. Concerning (ii), we find that the two vibrational levels with one quantum of torsional excitation are best described by a normal mode formulation, but that many levels with more than one quantum of torsional excitation are better described by a local-mode formulation. Concerning (iii), we obtain low-order molecular constants quite similar to those reported by Senent et al., but higher-order constants which are quite different. Our calculated spectrum agrees well with the low-resolution spectrum, but full confirmation of the present interpretation for levels with three or more quanta of torsion excited will require high-resolution studies of additional bands.

Gas phase electronic spectrum of the HSCCS radical by laser-induced fluorescence spectroscopy

In a discharged supersonic jet of a CS 2 and C 2H 2 mixture, a vibronic band system of a new radical species was observed in the energy region 21 800–23 000 cm −1 by laser-induced fluorescence (LIF) spectroscopy. The LIF excitation spectrum shows progressions with 490 and 80 cm −1 separations. The vibronic structure of a dispersed fluorescence (DF) spectrum, obtained by tuning a probe laser to the vibronic origin band, also consists of progressions with 520 and 100 cm −1 separations. A high-resolution laser scan provided a rotationally resolved LIF excitation spectrum for the vibronic origin band, showing the rotational structure of a-type transitions of a near-prolate top. Several chemical tests indicate that the spectral carrier contains sulfur atom(s), one hydrogen atom and more than one carbon atoms. Electronic transition energy, vibrational frequencies, and rotational constants of this species are similar to those of SCCS − [M. Nakajima, Y. Yoneda, Y. Sumiyoshi, T. Nagata, Y. Endo, J. Chem. Phys. 119(2003)7805–7813.], and the spectral carrier was assigned as an isoelectronic radical, bent HSCCS. Ab initio geometrical optimizations supported the spectral carrier to be HSCCS. The observed electronic transition was assigned to be the B ∼ 2 A ″ – X ∼ 2 A ″ transition, which corresponds to the Π–Π transition in the limit of linear geometry. The observed vibrations in the excitation and DF spectra were assigned as the symmetric C S stretching ( ν 5) and SCC bending ( ν 7) modes by comparing the results of theoretical calculations.

Impact of Extended π Conjugation on Methyl Rotor-Induced IVR in Aromatic Molecules

Laser-induced fluorescence excitation and resolved fluorescence spectra following excitations of the single vibronic levels (SVL) of p-vinyltoluene (p-VT) and p-vinylfluorobenzene (p-VFB) have been measured in a seeded supersonic free-jet expansion. A complete vibronic assignment of the fluorescence spectrum measured following excitation of the 0(0)0-band of p-VT has been presented. Normal vibrational modes in the S0 and S1 states of the molecule have been calculated by CASSCF method, and the correlation between the two set of modes is made by expressing the excited-state normal modes in terms of those of the ground state. The calculations predict that in the excited state methyl and vinyl torsional motions of p-VT are extensively mixed with many of the out-of-plane modes of the aromatic ring. Our resolved fluorescence spectral data measured following SVL excitations essentially agree with such predictions. In the excited state, the molecule exhibits a dramatically low threshold for the rotor-induced IVR in a supersonic jet expansion. Several mechanisms have been discussed to explain the phenomenon.

CF and CF2 radical kinetics and transport in a pulsed CF4 ICP

The kinetics of CF and CF2 radicals (in their electronic ground states) was investigated using space and time resolved laser induced fluorescence (LIF) in an inductively coupled discharge in CF4 gas at 33 mTorr, both in the steady state and in pulsed plasmas. The gas temperature was determined from the rotationally resolved LIF excitation spectra of the CF radical, reaching about 800 K in the centre of the chamber. The densities were put on an absolute scale with the aid of broad-band UV absorption spectroscopy. The density profiles were used to deduce the radical diffusive transport, and therefore the localized (gas-phase and surface) net production or destruction rates. However, when gas heating is significant, the radical transport becomes more complex due to gas density gradients and thermo-diffusion. In the afterglow, gas convection induced by gas cooling and contraction can play a significant role. The CF and CF2 axial fluxes estimated from measured density and temperature profiles indicate that these species are emitted from the top and bottom surfaces, and are destroyed in the gas phase at high rates (≈900 s−1 and 1500 s−1, respectively): for CF2 this process appears to be electron-impact induced. In the afterglow, the CF2 density at the reactor centre increases abruptly, reaching four times the steady-state value after 3 ms, followed by a slow decay (100 s−1). This phenomenon is the combined result of the return of CF2-rich gas as the central region cools and contracts, along with gas-phase reactions producing CF2. The central CF density shows only a slight initial increase in the afterglow, followed by a rapid decay (600 s−1), due to gas-phase chemical reactions, e.g. with F2 or CF3, converting it to CF2, processes which are also likely to be important in the steady state.

Absolute radical concentration measurements in low-pressure H 2/O 2 flames during the combustion of graphite

Absolute CN and CH radical concentrations were determined in situ during the combustion of a graphite substrate in premixed, laminar, low-pressure, H 2/O 2 flames for two different equivalence ratios, ϕ = 1.0 and ϕ = 1.5. For CN measurements, a small amount of NO (1.8%) was added. The concentration of CN was measured by cavity ring-down spectroscopy (CRDS) probing the absorption of the P 1,2 (13) in the B–X (0, 0) band at 388.1 nm, and the concentration of CH was measured by linear unsaturated laser-induced fluorescence (LIF) exciting the fluorescence of the R 1 (4) in the B–X (0, 0) band at 387.4 nm. Temperature measurements were done based on LIF excitation spectra of OH in the A–X (0, 0) band. It was found that the graphite substrate reduces the flame temperature in the vicinity of its surface. The CN concentrations were found to be three times higher for the rich flame than for the stoichiometric flame. CH concentrations were slightly higher for the stoichiometric flame than for the rich flame. The observed CH/CN concentration ratio is substantially lower compared to NO-doped low-pressure CH 4/O 2 flames. The obtained quantitative information can serve as a first calibration point for detailed numerical simulations of the burning solid graphite, which are based on the concept of surface elementary reactions.

Dispersed fluorescence spectroscopy of primary and secondary alkoxy radicals

Dispersed fluorescence (DF) spectra of 1-propoxy, 1-butoxy, 2-propoxy, and 2-butoxy radicals have been observed under supersonic jet cooling conditions by pumping different vibronic bands of the B-X laser induced fluorescence excitation spectrum. The DF spectra were recorded for both conformers of 1-propoxy, three conformers of the possible five of 1-butoxy, the one possible conformer of 2-propoxy, and two conformers of the possible three of 2-butoxy. Analysis of the spectra yields the energy separations of the vibrationless levels of the ground X and low-lying A electronic state as well as their vibrational frequencies. In all cases, the vibrational structure of the DF spectra is dominated by a CO stretch progression yielding the nuCO stretching frequency for the X state and in most cases for the A state. In addition to the experimental work, quantum chemical calculations were carried out to aid the assignment of the vibrational levels of the X state and for some conformers the A state as well. Geometry optimizations of the different conformers of the isomers were performed and their energy differences in the ground states were determined. The results of the calculation of the energy separations of the close-lying X and A states of the different conformations are provided for comparison with the experimental observations.

Fine structure of the (S1←S) band origins of phthalocyanine molecules in helium droplets

The laser-induced fluorescence (LIF) excitation spectra of free base phthalocyanine (Pc), Mg-Pc, and Zn-Pc molecules in superfluid helium droplets at T=0.38 K have been studied. The spectra reveal the rich vibronic structure of the S(1)<--S(0) electronic transitions. The band origins of the transitions consist of zero phonon lines accompanied by phonon wings, which originate from simultaneous electronic excitation of the molecule and excitation of the collective modes of the helium surrounding it. The phonon wings have discrete structures suggesting localization of some helium atoms in the neighborhood of the molecules. Zero phonon lines of Mg-Pc and Zn-Pc molecules are split into three components, which are separated by 0.2-0.4 cm(-1). Possible mechanism of splitting involves static or dynamic Jahn-Teller interaction of metal-phthalocyanine molecules in the twofold degenerate S(1)((1)E(u)) state with the helium shell.

The rotationally resolved electronic spectra of several conformers of 1-hexoxy and 1-heptoxy

Laser-induced fluorescence excitation spectra of five vibronic bands of 1-hexoxy and three bands of 1-heptoxy have been recorded in a jet-cooled environment. Experimental values of rotational constants for both the [Formula: see text] and [Formula: see text] states and components of the spin-rotational tensor for the [Formula: see text] state were obtained by an analysis of the partially resolved rotational structure of the vibronic bands. Comparing these experimental results with quantum chemistry calculations, and using corresponding assignments of smaller alkoxy radicals as a guide, permitted unambiguous conformational assignments for the bands. The extension of similar assignments to larger alkoxy radicals is also discussed. Key words: electronic spectroscopy, organic radicals, combustion, atmospheric chemistry.

Photodissociation spectroscopy and dynamics of the CH2CFO radical.

The photodissociation spectroscopy and dynamics resulting from excitation of the B (2)A(")<--X (2)A(") transition of CH(2)CFO have been examined using fast beam photofragment translational spectroscopy. The photofragment yield spectrum reveals vibrationally resolved structure between 29 870 and 38 800 cm(-1), extending approximately 6000 cm(-1) higher in energy than previously reported in a laser-induced fluorescence excitation spectrum. At all photon energies investigated, only the CH(2)F+CO and HCCO+HF fragment channels are observed. Both product channels yield photofragment translational energy distributions that are characteristic of a decay mechanism with a barrier to dissociation. Using the barrier impulsive model, it is shown that fragmentation to CH(2)F+CO products occurs on the ground state potential energy surface with the isomerization barrier between CH(2)CFO and CH(2)FCO governing the observed translational energy distributions.

Laser-induced fluorescence spectroscopy of NC3S.

In a discharged supersonic jet of acetonitrile and carbon disulfide, we have for the first time observed an electronic transition of the NC(3)S radical using laser-induced fluorescence (LIF) spectroscopy. A progression originating from the C-S stretching mode of the upper electronic state appears in the excitation spectrum. Each band of the progression has a polyad structure due to anharmonic resonances with even overtones of bending modes. Rotationally resolved spectra have been observed by high-resolution laser scans, and the electronic transition is assigned to A 2Pii-X 2Pii. For the vibronic origin band, the position and the effective rotational constant of the upper level have been determined to be 21 553.874(1) and 0.046 689(4) cm(-1), respectively. The dispersed fluorescence spectrum from the zero vibrational level of A 2Pi3/2 has also been observed; its vibrational structure is similar to that of the LIF excitation spectrum, showing a prominent C-S stretching progression with polyad structures. The vibrational frequencies of the C-S stretching mode in the ground and excited electronic states are determined to be 550 and 520 cm(-1), respectively. Fluorescence decay profiles have been measured for several vibronic levels of the A state.

Laser spectrometry and kinetics of selected elementary reactions of the acetonyl radical

The laser induced fluorescence excitation spectrum (LIF) and the ultraviolet absorption spectrum (TA) of the acetonyl radical (CH3C(O)CH2) were remeasured by using the time-resolved fast discharge flow (DF) and laser flash photolysis (LP) experimental techniques (T = 298 K). The absorption cross section of σ(acetonyl, 207 nm) = (3.16 ± 0.61) × 10−18 cm2 molecule−1 was determined calibrated against the acetyl-peroxyl radical (CH3C(O)O2) in LP/TA measurements. The kinetics of the reactions of CH3C(O)CH2 with the open shell reaction partners O2 (1), NO (2), NO2 (3) and H (4) were studied by using the DF method with LIF detection of the acetonyl radical at 298 ± 1 K and 2.85 ± 0.05 mbar He pressure. The rate constants for the overall reactions were determined in units of cm3 molecule−1 s−1 to be k1 = (3.49 ± 0.51) × 10−13, k2 = (1.04 ± 0.19) × 10−11, k3 = (3.25 ± 0.65) × 10−11 and k4 ≥ 3 × 10−10 with 2σ accuracy given. The acetonyl radical was found to react similarly to alkyl radicals by comparison with literature results. A reduced reactivity was observed toward O2 and NO that might be attributed to the resonance stabilisation of the acetonyl radical. No such effect was observed for the NO2 and H atom reactions.

LIF Spectra of Cyclohexoxy Radical and Direct Kinetic Studies of Its Reaction with O2

The laser-induced fluorescence (LIF) excitation spectrum of cyclohexoxy radical has been measured for the first time. The dominant vibrational progression is consistent with computations, at CIS/6-31+G(d), of the C−O stretch frequency of the axial conformer of cyclohexoxy radical. LIF intensity was used as a probe in direct kinetic studies of the reaction of cyclohexoxy radicals with O2. The Arrhenius expression obtained was kO2 = (5.8 ± 2.3) × 10-12 exp[(−14.3 ± 0.8) kJ/mol/RT] cm3 molecule-1 s-1 (225−302 K), independent of pressure in the range 50−125 Torr. The room temperature rate constant for this reaction is a factor of 2 higher than the commonly recommended value, but the observed activation energy is 9 times larger than the recommended value of 1.6 kJ/mol. Combining our results with the ratio of rate constants, kO2/kscission, measured in chamber experiments, we obtained an Arrhenius expression for kscission, the rate constant for β C−C scission of cyclohexoxy radical. However, the resulting Arrhenius preexponential factor of 4.5 × 1015 s-1 is unreasonably high compared with the value of ∼2 × 1013 s-1 obtained in our RRKM/Master Equation calculations as well as in calculations and experiments reported for other alkoxy radicals. The apparent discrepancy is resolved by examining the uncertainties in the values of kscission and the limited temperature range spanned by the relative rate experiments. A part of the discrepancy might also be explained by the observation that the O2 rate constant measured here is only for a single conformer of cyclohexoxy radical, whereas the relative rate experiments represent some averaging over both conformers.

Laser-induced fluorescence excitation spectra due to the [formula omitted] transition of NO in magnetic fields up to 10 T

Laser-induced fluorescence (LIF) excitation spectra due to the B 2 Π(v ′′=0)–X 2 Π(v ′=0) transition of nitric oxide (NO) were measured in magnetic fields, H, ranging from 0⩽ H⩽10 T. Rotational lines were clearly observed in the spectra, although the transition intensity is very weak because of the small Franck–Condon factor of transition. All observed lines were assigned based on quantum mechanical calculations. Calculated transition intensities agreed well with the observed intensities, indicating that previously reported magnetic field quenching in the B 2 Π(v ′′=0) state does not occur in the observed energy region ( J ′′⩽15.5). Consequently, it was concluded that the magnetic quenching in the B 2 Π(v ′′=0) state occurs in the rotational levels with J ′′>15.5.

Radiative and non-radiative decay of selected vibronic levels of the [formula omitted] state of alkoxy radicals

We have measured the lifetimes of some of the prominent bands observed in the moderate-resolution, jet-cooled laser induced fluorescence excitation spectra of the B ← X transition of ethoxy, all the structural isomers of propoxy and butoxy, and 1-pentoxy. Recent high-resolution, rotationally resolved studies on primary alkoxy radicals have given conformer specific assignments for the bands for which lifetimes have been measured. We report observed lifetime trends as a function of vibrational excitation for specific isomers and conformers. The implications of these observations for the dynamics of the B state will be discussed.

Laser spectroscopy of jet-cooled CuF in visible region

The laser-induced fluorescence excitation spectra of jet-cooled CuF have been recorded in the range of 19000-21470cm-1, in which the CuF radicals were produced by the reaction of SF6 with copper atoms from a dc discharge-sputtering source under supersonic jet conditions. Eight observed vibronic transition bands have been assigned as the transition from the ground state X(1Σ+) to B(1Σ+), C(1Π) and an unreported upper state. The rotational structure of all observed bands has been analysed at the estimated rotational temperature 80K. We determined the newly observed band to be the (1Π,v = 1)-X(1Σ+v = 0) transition according to the character of the rotational structure and the isotopic shift analysis. In addition, the lifetimes of the states involved in these bands were measured.

Visible laser spectroscopy of cobalt monofluoride

The laser-induced fluorescence excitation spectrum of jet-cooled CoF molecules has been studied in the range of 18 800–22 000 cm −1. Ten observed vibronic bands have been classified into three transitions with the 0–0 band at 18 909, 19 236, and 20 654 cm −1, assigned as the [18.8] 3Φ 4– X 3Φ 4, [19.2] 3Φ 4– X 3Φ 4, and [20.6] 3Γ 5– X 3Φ 4 transition, respectively, the two 3Φ states, [18.8] 3Φ and [19.2] 3Φ, are consistent with Adam’s results (10). The previously unanalyzed [20.6] state is identified in the current work. A rotational analysis of [20.6] 3Γ 5– X 3Φ 4 transition has been performed and effective equilibrium molecular constants have been determined for the first time. In addition, lifetime measurements of the three electronic transitions were carried out under the collision-free condition. From the lifetime analysis, we consider that the V=1, 2, and 3 vibrational levels of [18.8] 3Φ state are perturbed by another state.

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 6a 1 band, while only the 3a 1 band in addition to the 0a 1 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.

Structure and electronic spectroscopy of naphthalene–acenaphthene van der Waals dimer: Hole-burning, dispersed fluorescence, and quantum chemistry calculations

Electronic spectroscopy of 1:1 van der Waals dimer of naphthalene and acenaphthene has been studied in a supersonic free jet expansion by measuring the laser-induced fluorescence excitation, dispersed fluorescence, and two-color hole-burning spectra. In fluorescence excitation spectrum the dimer exhibits a long progression of an intermolecular vibration, and similar vibronic structures are observed also in emission spectra from the origin region of the S1 surface. The excimer formation from the locally excited state appears as a barrier crossing process and excess vibronic energy required to cross the barrier is about 420 cm−1. The equilibrium structure and binding energy of the dimer are computed by the ab initio quantum chemistry method at the MP2/6-31G and MP2/6-31+G*//MP2/6-31G levels. A parallel-displaced structure, in which two molecules are displaced from a fully overlapping geometry by 1.16 and 0.45 Å, respectively, along the long and short molecular axes, and maintains a vertical separation of 3.48 Å between two molecular planes, is found to be the most stable in the ground state. The BSSE corrected MP2/6-31+G*//MP2/6-31G binding energy of the dimer is 9.2 kcal/mol. The observed spectral and dynamical characteristics of the mixed dimer are compared to those reported for the naphthalene homodimer, and the differences are interpreted in terms of geometry and exciton resonance interactions.

Laser-induced fluorescence excitation spectra of 1,4-Di(1-naphthyl)propane and 1-buthylnaphthalene in a supersonic jet

Laser-induced fluorescence excitation spectra of the bichromophoric molecule, 1,4-Di(1-naphthyl)propane (N4N(1,1′)), and of 1-buthylnaphthalene (1-BuN ) in a supersonic jet are reported. Comparison between the two spectra suggests that the two naphthalene moieties in the bichromophoric molecule do not interact strongly. Exciton splitting or evidence for excimer formation was not observed. However, the aliphatic substitution alone, cannot account for the significant red shift of the origin in the spectrum of the bichromophoric molecule.