J.A. Fernández

Isomer structures and vibrational assignment of the methyl-p-aminobenzoate(H2O)1 complex

Methyl-p-aminobenzoate(H2O)1 complex, henceforth MAB(H2O)1, prepared by pulsed supersonic expansion, has been examined by a broad range of laser based spectroscopic, mass and isomer selective techniques and density functional theory (DFT) calculations, in order to identify its isomer structures, ionization energies, and vibrational frequencies. The experimental techniques used include laser induced fluorescence (LIF), mass resolved excitation spectroscopy (MRES) either with one (REMPI) or two laser colors (R2PI), laser excited dispersed emission (DE), high resolution MRES, pressure controlled R2PI, hole burning (HB) spectroscopy, and photoion fragmentation threshold (PIFT). Experimental results have been interpreted, rationalized and extended with density functional theory (DFT) computations at the B3LYP/6-31G and B3LYP/6-31+G* levels. Although bare MAB molecule have four possible solvation sites, prone to yielding hydrogen bonds with the water molecule, LIF, R2PI, and HB spectroscopy of MAB(H2O)1 only pi...

GROUND AND FIRST ELECTRONIC EXCITED STATE VIBRATIONAL MODES OF THE METHYL-P-AMINOBENZOATE MOLECULE

Abstract Laser induced fluorescence, dispersed emission and two-colour ionisation spectroscopy (including the ionisation energy threshold) are suggested as an appropriate set of experimental methods for drawing the ground and excited state vibrational spectra of large organic molecules in presence of solvated molecules and dimers. In this Letter, they have been applied to obtain the vibrational spectrum of methyl- p -aminobenzoate (MAB). Parallel ground state ab initio calculations have enabled the geometry to be determined and the vibrational modes to be accurately assigned. The comparison of the ground and the first electronic excited state vibrational band wavenumbers shows an excellent agreement except for the excited state bands observed at 52.2, 565.6 and 579.7 cm −1 .

Removal rates of CHF (Ã 1A″ (0, 0, 0)) by alkenes

Abstract Absolute removal rates of CHF (A 1 A″ (0, 0, 0)) by ethene (C 2 H 4 ), propene (C 3 H 6 ), 1-butene (1-C 4 H 8 ), isobutene( i -C 4 H 8 ), 1,3-butadiene (C 4 H 6 ), difluoromethane (CH 2 F 2 ), nitric oxide (NO) and argon (Ar) have been measured at room temperature. CHF in the A 1 A″ state was produced by infrared multiphoton dissociation of CH 2 F 2 forming the CHF (X 1 A′) state and further pumping to the A 1 A″ state by absorption of a visible dye laser pulse. Removal processes were found to be second order with the following rate constants in units of 10 −10 cm 3 molecule −1 s −1 : k (C 2 H 4 ) = 0.9 ± 0.2. k (C 3 H 6 ) = 1.0 ± 0.2; k (1−1C 4 H 8 ) = 1.1 ± 0.2; k ( i -C 4 H 8 ) = 1.1 ± 0.2; k (C 4 H 6 ) = 1.0 ± 0.2; k (Ar) = 0.27 ± 0.02; k (NO) = 0.8 ± 0.1; k (CH 2 F 2 ) = 1.3 ± 0.1. The Parmenter—Seaver correlation for collisional removal of A 1 A″ CHF is discussed.

Experimental and theoretical study of methyl-p-aminobenzoate/ammonia complexes. I. MAB(NH3)1

A complementary laser spectroscopy and computational study of the MAB(NH3)2–4 complexes, hereafter referred to by its stoichiometry, i.e., 1:2, 1:3, and 1:4, prepared in a supersonic expansion, is reported. Experimental evidence shows the existence of abundant fragmentation cascades, the most notorious being the observation of the 1:4 complex spectrum in the 1:3 and to 1:2 mass channels, in fact, the observed spectra of the 1:2 and 1:3 complexes are not genuine but a consequence of fragmentation. The observed 1:4 complex resonance enhanced multiphoton ionization (REMPI) spectrum has a significant redshift of −1160 cm−1 from the bare MAB 000 transition and appears over a noisy background that decreases, although it does not disappear, in resonance enhanced two-color photo ionization (R2PI) studies. “Hole burning” spectroscopy corroborates the presence of only one 1:4 isomer. Calculations at the B3LYP/6−31+G* level conduct to a number of 1:2, 1:3, and 1:4 stable isomer structures, the most stable being the ...

Collisionally induced intramultiplet mixing of metastable states by Ne, Kr and Xe atoms

Abstract An investigation is presented of the spin–orbit mixing within the Sr [5 s5p ( 3 P 0,1,2 )] manifold on collision with Ne, Kr and Xe at elevated temperatures following pulsed dye-laser generation of Sr ( 3 P 1 ) . The population profiles of all three states were monitored individually by laser-induced fluorescence and fitted to a kinetic model involving six collisional rates connected by detailed balance. The model also includes mixing by Sr ( 1 S 0 ) itself. The resulting rate constants, together with those for Sr ( 1 S 0 ) , Ar and He reported previously, are considered using a standard model employing the potential wells of the reactants and also a collision-complex model.

Direct measurements of removal rates of electronically excited NCO(Ã2Σ+(0,0,0)) by simple molecules

Abstract Direct measurements of absolute rate constants are reported for the collisional removal at room temperature of NCO ( A 2 Σ + (0,0,0)) by O 2 ( 3 Σ g ), N 2 ( 1 Σ + g ), NO ( 2 Π), CO 2 ( 1 Σ + g ), N 2 O ( 1 Σ + g ), SO 2 ( 1 A ) and the photochemical precursor itself, phenyl isocyanate, PhNCO. NCO ( X 2 Π 3 2 (0,0,0)) was prepared by infrared multiphoton dissociation (IRMPD) of PhNCO followed by tuneable dye laser pumping at the transition NCO ( A 2 Σ + (0,0,0)) ← X 2 Π 3 2 (0,0,0)) coupled with laser-induced fluorescence (LIF) monitoring in the time-domain. Removal was found to be very efficient where the rate constants with the above gases were measured as 2.05±0.2, 15.2±0.2, 32.0±3.0, 8.52±0.4, 6.46±0.2, 27.1±1.0 and 51.8±5.0, respectively, in units of 10−11 cm3 molecule−1 s−1 (at a laser fluence of 43 J/cm2). An upper limit of the rate constant for the physical quenching of NCO(A) by Ar is also reported as (ϵ/k) 1 2 ) of the collision partners, consequently yielding a pair well depth ( (ϵ/k) 1 2 ) of 94±15 K 1 2 for NCO ( A 2 Σ(0,0, 0)) .

Collisionally induced intramultiplet mixing of Sr(53PJ) metastable states by He, Ar and Sr ground state atoms

Abstract Intramultiplet collisionally induced mixing within the Sr[5s5p(3P0,1,2)] manifold is investigated by time-resolved laser-induced fluorescence (LIF) following the pulsed dye-laser generation of Sr(3P1) of the electronic ground state {Sr[5s5p(3P0,1,2)]←Sr[5s2(1S0)], λ=689.26 nm} at elevated temperatures. The population profiles of the three spin–orbit states were individually monitored by LIF as well as that of Sr(3P1) by spontaneous emission at the resonance wavelength. A kinetic model is employed that enables the process of spontaneous emission from Sr(3P1) to be isolated initially and characterised by experiment. Particular emphasis is placed on the modelling procedure itself in which the separate kinetic component due to spontaneous emission and the positions of the maxima in the 3P0 and 3P2 population profiles constitute severe constraints on the model. The collisional components within the model are reduced to three rate constants where pairs of J states are connected in this context by detailed balance. Thus k10 and k12, and, by detailed balance, k01 and k21 are quantified at various temperatures and pressures to yield the absolute value of these collision properties for Sr(3PJ) with He and Ar. Rate data for collisionally induced intramultiplet mixing in Sr(3PJ) by Sr(1S0) itself is also reported found to proceed at close to unit collisional efficiencies in all cases. Thus, at elevated temperatures, variations in atomic profiles are dominated by the differing vapour pressures of atomic strontium. Estimates of the activation energies associated with k10 and k12 for the noble gases observed against this large competing background are found to be of the order of the spin–orbit splittings. The model overall is found to be insensitive to k02 and k20 whose magnitudes are small by comparison with those for the collisional rate data connecting adjacent J states. Whilst collisional processes for He are some two orders of magnitude more efficient than those for Ar, all of these are seen to be ‘adiabatic’, in contrast with the gas kinetic rate constants of ground Sr, considered to be ‘sudden’ in character. The results are compared with analogous data derived by atomic resonance absorption spectroscopy following pulsed generation of Sr(3P1) and are considered in the context of theoretical calculations employing quantum close coupling calculations.

Atomic fluorine emission cross sections observed in the 600-900 nm region following 0-500 eV electron impact on fluoromethanes (CF4, CHF3, CH2F2 and CH3F)

Absolute emission cross sections (em) of F I atom electronic transitions in the 600-900 nm region, following the electron impact on CHxF4-x (x = 0-3) molecules and determined by the optical method, are reported. The range of electron energies studied varies from threshold to 500 eV. In the electron impact on CF4 at pressures higher than 2 mTorr, a continuum background band in the 500-700 nm region is observed and has been assigned to the CF3* (2A2´´1A1´ and/or 1E´1A1´) transitions. Continuum bands of the same features were also found in collisional processes on CHF3 and CH2F2, although their maxima are shifted to the blue with respect to that of CF4 and have been tentatively assigned to the F2* ion pairs. For low F content fluoromethanes high electronic excited states (3d) of atomic F are observed and may be due to redistribution of the excess electron energy over that for C-F bond dissociation.

Direct measurements of removal rates of CFCl(Ã1A″(0, 1, 0)) by simple alkenes

Abstract Direct measurements of rate constants for the collisional removal of CFCl(A 1 A″(0, 1, 0)) by ethylene (C 2 H 4 ), propene (C 3 H 6 ), 1-butene (1-C 4 H 8 ), i -butene ( i -C 4 H 8 ) and 1,3-butadiene (C 4 H 6 ) are reported. CFCl(A 1 A″(0, n , 0)) was prepared by infrared multiple photon dissociation (IRMPD) of CHFCl 2 followed by tunable dye laser pumping at the transition CFCL( A 1 A ″(0, 1, 0) ← X 1 A ″(0, 1, 0)) . Removal rates at threshold CO 2 -laser fluence for the gases ethylene, propene, 1-butene, 1,3-butadiene and i -butene were measured to be 1.4 ± 0.15, 3.7 ± 0.7, 2.2 ± 0.2, 4.0 ± 0.8 and 4.1 ± 0.4 in 10 −11 cm 3 molecule −1 s −1 units. An upper limit of the rate constant for physical quenching of CFCl(A) by Ar is also reported as −13 cm 3 molecule −1 s −1 . The rate date are compared with those for reactions of other carbenes and the appropriate Parmenter-Seaver plot is presented.

KINETIC AND SPECTROSCOPIC STUDIES OF TRANSIENT SPECIES PREPARED BY IRMPD

Infrared multiple photon absorption is known to be observed in some triatomic and most polyatomic molecules. When the number of absorbed photons is high enough the process leads to photodissociation and excess energy is released as vibrational, rotational and translational energy in the electronic ground state of the fragments. The contribution of these energies is determined by standard methods, that include laser induced fluorescence, LIF, and time-resolved Fourier transform spectroscopy, TR-FTS. Examples of selected determinations are shown.