Overtone Excitation Research Articles

Local modes of HOOH probed by optical-infrared double resonance

We have used an optical-infrared double resonance technique to probe the nature of the eigenstates prepared by 4νOH vibrational overtone excitation in hydrogen peroxide. A visible dye laser excites the 4←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=2 transition from the 4νOH level. Fixing the overtone excitation laser wavelength and scanning the wavelength of the infrared photon while monitoring the dissociation fragments by laser-induced fluorescence generates an infrared predissociation spectrum of the vibrationally excited molecule that contains information about vibrational state mixing at the 4νOH level. This spectrum indicates that the zeroth-order state that gives oscillator strength to the 4←0 OH stretch transition (i.e., the 4νOH bright state) is almost entirely comprised of a single vibrational eigenstate. Since the bright state is predominantly an OH stretch, the vibrational eigenstate prepared by 4νOH vibrational overtone excitation is well localized on the OH bond. This localization allows us to perform sequential local mode–local mode excitation of the two equivalent OH oscillators in HOOH.

Controlling bimolecular reactions: Mode and bond selected reaction of water with hydrogen atoms

Vibrational overtone excitation prepares water molecules in the ‖13〉−, ‖04〉−, ‖12〉−, ‖02〉−‖2〉, and ‖03〉− local mode states for a study of the influence of reagent vibration on the endothermic bimolecular reaction H+H2O→OH+H2. The reaction of water molecules excited to the ‖04〉− vibrational state predominantly produces OH(v=0) while reaction from the ‖13〉− state forms mostly OH(v=1). These results support a spectator model for reaction in which the vibrational excitation of the products directly reflects the nodal pattern of the vibrational wave function in the energized molecule. Relative rate measurements for the three vibrational states ‖03〉−, ‖02〉−‖2〉, and ‖12〉−, which have similar total energies but correspond to very different distributions of vibrational energy, demonstrate the control that initially selected vibrations exert on reaction rates. The local mode stretching state ‖03〉− promotes the H+H2O reaction much more efficiently than either the state having part of its energy in bending excitation (‖02〉−‖2〉) or the stretching state with the excitation shared between the two O–H oscillators (‖12〉−). The localized character of the vibrational overtone excitation in water has permitted the first observation of a bond selected bimolecular reaction using this approach. The reaction of hydrogen atoms with HOD molecules excited in the region of the third overtone of the O–H stretching vibration, 4νOH, forms at least a 100-fold excess of OD over OH, reflecting the preferential cleavage of the vibrationally excited bond.

Infrared spectroscopy of vibrationally excited HONO2: Shedding light on the dark states of intramolecular vibrational energy redistribution

Infrared predissociation spectroscopy of nitric acid subsequent to vibrational overtone excitation reveals vibrational state mixing of the highly excited levels and probes the character of the coupled dark states. A visible dye laser excites the 4←0 or 5←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=1 transition from the excited level. Scanning the optical parametric oscillator frequency while monitoring the predissociation products via laser-induced florescence produces an infrared spectrum of the excited molecules. Although the 4νOH vibrational overtone band consists of a single clean rotational contour that falls directly on a Birge–Sponer plot, the infrared transitions from this level indicate that the zeroth-order bright state is extensively mixed. On the average, the zeroth-order bright state is only a minor component of the eigenstates at this energy. The largest collective contribution is from zeroth-order states that have zero quanta of OH stretch.

Broad vibrational overtone linewidths in the 7νOH band of rotationally selected NH2OH

Infrared–optical double-resonance spectroscopy of the 7νOH vibrational overtone level of NH2OH reveals 14 cm−1 wide spectral features. The product state distribution of the OH fragment subsequent to overtone excitation indicates that the 7νOH level of NH2OH is ∼128 cm−1 above the N–O bond dissociation energy. Comparison to HOOH overtone spectra at a similar excess energy suggests that the broad NH2OH linewidths result from vibrational state mixing at the 7νOH level and not from inhomogeneous structure or lifetime broadening of the dissociating molecules. The observation of 14 cm−1 overtone linewidths for a molecule the size of NH2OH suggests that the broad vibrational overtone transition linewidths in larger polyatomics may contain a substantial homogeneous component.

Collision dynamics of OH(X 2Πi,v=12)

Collisional removal rate constants for the OH radical in v=12 of the ground electronic state are measured for the colliders CO2, O2, N2, H2, He, and Ar. OH molecules, generated in v=8 by the reaction of hydrogen atoms with ozone, are excited to v=12 by direct overtone excitation with pulsed infrared laser light. The temporal evolution of the v=12 radicals is probed as a function of collider gas pressure by a time-delayed pulsed ultraviolet probe laser. The probe laser is used to excite the molecules via the B 2Σ+–X 2Πi(0,12) electronic transition, and the resulting B 2Σ+–A 2Σ+ fluorescence is detected. We measure rate constants for CO2:(5.6±1.5)×10−11; O2:(1.6±0.2)×10−11; He:(3.6±0.6)×10−12; H2:(3.0±0.8)×10−12; Ar:(2.6±0.5)×10−12; N2:(2.5±0.7)×10−12 (all in units of cm3 s−1). These rate constants are over fifty times faster in all cases than the vibrational relaxation rate constants for the lower levels (v=1 and v=2) of the ground state.

Laser double-resonance study of OH ( X2Π i, v = 12)

The spectroscopy of OH ( X 2Π i , v = 12) is examined using a two-laser double-resonance method. OH molecules generated in v = 8 by the reaction of hydrogen atoms with ozone are excited by pulsed tunable infrared laser light via direct overtone excitation to v = 12. The absorption is monitored either by a depletion in B 2Σ +- X 2Π i laser-induced fluorescence (LIF) signals on the (0, 8) band or by an observation of LIF signals on the B- X (0, 12) band. Line positions for X- X (12, 8) and B- X (0, 12) are reported. The relevance of these measurements to spacecraft glow and ab initio theoretical calculations is discussed.

Vibrational energy transfer in OH X 2Πi, v=2 and 1

Using an infrared pump/ultraviolet probe method in a flow discharge cell, vibrational energy transfer in OH X 2Πi has been studied. OH is prepared in v=2 by overtone excitation, and the time evolution of population in v=2 and 1 monitored by laser-induced fluorescence. Rate constants for vibrational relaxation by the colliders H2O, NH3, CO2, and CH4 were measured. Ratios of rate constants for removal from the two states, k2/k1, range from two to five.

Bond-selected bimolecular chemistry: H+HOD(4νOH)→OD+H2

The reaction of HOD containing four quanta of O–H bond stretching vibration with H atoms produces OD fragments almost exclusively. Vibrational overtone excitation prepares HOD(4νOH) that reacts with H atoms formed in a microwave discharge. The endothermic reaction of water with hydrogen atoms does not occur for ground vibrational state water but proceeds at roughly the gas kinetic collision rate for the vibrationally excited molecule. The production of OD fragments from HOD(4νOH) in the reaction is at least two orders of magnitude more efficient than the production of OH, indicating very selective reaction of the vibrationally excited bond.

Bimolecular reaction of a local mode vibrational state: hydrogen atom + water (4.nu.OH) .fwdarw. hydroxyl(v,J) + hydrogen

Vibrational overtone excitation of water to the {vert bar}04>{sup {minus}} local mode state is used to initiate the endothermic bimolecular reaction of hydrogen atoms with water molecules. A probe of the nascent OH product using laser-induced fluorescence indicates that this fragment receives less than 10% of the {approximately}9,000 cm{sup {minus}1} of available energy consistent with a spectator model. Most of the OH fragments ({approximately}98%) are formed in the vibrational ground state with an average of 360 cm{sup {minus}1} of rotational energy and {approximately}350 cm{sup {minus}1} of translational excitation. The present results on the H + H{sub 2}O system demonstrate the feasibility of using vibrational overtone excitation to promote bimolecular reactions involving reagents containing light hydrogen atom stretching motion and, thus, investigate the influence of local mode excitation on chemical reactivity.

A molecular beam time-of-flight mass spectrometer using low-energy-electron impact ionization

We have constructed a versatile apparatus to study photoinitiated processes in molecular beams using a variety of generally applicable techniques. The instrument contains a pulsed, low-energy electron gun that delivers space-charge-limited electron beams into the ionization region of a time-of-flight mass spectrometer. The electron energy is tunable from 8 to 200 eV, and the electron energy distribution is relatively narrow (FWHM ∼0.3 eV), which allows us to ionize laser-excited species and their decomposition products selectively. We have used low-energy electron impact ionization and mass spectrometry to characterize molecular beams, to detect vibrationally excited molecules prepared by vibrational overtone excitation, and to detect primary photodissociation products in the presence of precursor molecules.

On the possibility of prolonging the lifetime of CH overtones through double-resonance excitation

On the possibility of prolonging the lifetime of C-H overtones through double- resonance excitation A. Lami and G. Villani Istituto di Chimica Quantistica ed Energetica Molecolare de1 CNR Via Risorgimento, 35 - 56100 Pisa (Italy) The possibility of utilizing laser sources to promote a given chemical reaction is entirely based on the hope that in this way one can prepare and mantain a sufficient population in an excited state satisfying the following two requirements: i) it must belong to the reactant valley in order to be accessible with a non negligible probability by photon absorption from the ground state (due to the Franck-Condon principle); ii) it must exhibit a propensity to evolve in to the product valley. Condition ii) is the most difficult to be met and the usual fate of an excited state in the reactive region of a polyatomic is to rapidly become a more or less ergodic misture of all the excited states having the same energy, on the reactant side. Once this has occurred, every selectivity is lost and the products yield is completely determined by statistics. Despite the hardness of the task, several theoretical schemes have been proposed l-l 11 to attain a laser control of selected reactions. Our proposal [ll] was to use a double-resonance irradiation, where one frequency is used to excite the selected state and the other one to modify the internal dynamic leading to “ergodization”. Here we reconsider this “coherent trapping” scheme and show that it can be used to make the lifetime of a given C-H overtone longer, thus enhancing the rate of reactions involving tunneling from a C-H group 19,121. The hydrogen tunneling reactions seem to us to be among the best candidates to exhibit laser induced “mode selectivity” since, by overtone excitation one may excite directly the vibration that goes into the reaction coordinate. In the next section we expose the theoretical basis of what we called “coherent trapping control”

Dissociation of remote bonds by overtone excitation: A model study of heavy-atom blocking

We present a classical-mechanical study of model systems which allow the rupture of a weak bond subsequent to excitation of overtones in remote parts of the molecule. Our models are the linear chains HCCCX and HCMCX, where M is a heavy atom and the CX bond is weak. The ensuing mode-specificities and nonstatistical behavior are strongly affected by the presence of a heavy-atom blocker.

Mode specificity and the influence of rotation in cis-trans isomerization and dissociation in HONO

The results of a classical trajectory study of intramoleeular vibrational energy redistribution, cis-trans isomerization, and unimolecular dissociation in HONO are presented. The calculations were carried out on a realistic potential-energy surface that was constructed by using the available kinetic, thermochemical, spectroscopic, and ab initio quantum mechanical information. The influence of the total energy, initial normal-mode excitations, initial OH-stretch overtone excitations, rotation, and potential-energysurface on intramolecular vibrational redistribution and the initial rates of isomerization and dissociation is discussed. The results show significant mode-specific behavior, particularly for the isomerization. Excitations of overtones of the OH or NO bond stretching modes yield the lowest initial rates for both isomerization and dissociation. Excitation of the HON bending mode yields the largest isomerization rates while excitation of the ONO bending mode yields the largest dissociation rate. At a fixed total energy, placing a small amount of rotational energy in the molecule causes a significant increase in the isomerization and dissociation initial rates over those computed for nonrotating HONO, however, when the rotational energy is increased above 0. 1 eV, the rates decrease as expected on the basis of RRKM theory. The orientation of the rotation is an important factor for the intramolecular energy transfer and reaction rates. Rotating the molecule along the torsional axis causes a significant increase of the initial rate of isomerization. Rotating the molecule perpendicular to the ON bond causes significant increases in the dissociation rates.

The vibrationally mediated photodissociation dynamics of nitric acid

Vibrationally mediated photodissociation, in which one photon prepares a highly vibrationally excited molecule by vibrational overtone excitation and a second photon dissociates the vibrationally excited molecule, is a means of studying the spectroscopy and photodissociation dynamics of highly vibrationally excited states. Applying this dissociation scheme to nitric acid (HONO2) excited in the region of the third overtone of the O–H stretching vibration (4νOH) and detecting the OH fragment by laser induced fluorescence determines the energy partitioning and identifies the influence of vibrational excitation prior to dissociation. Vibrationally mediated photodissociation using 755 and 355 nm photons deposits more energy in relative translation than the isoenergetic single photon dissociation with 241 nm light. The former process also produces three times more vibrationally excited OH fragments, and both processes form electronically excited NO2, which receives over three-quarters of the available energy. In these experiments, vibrational overtone excitation enhances the cross section for the electronic transition by about three orders of magnitude. The observed differences are consistent with the motion of the vibrationally excited molecule on the ground electronic state surface strongly influencing the dissociation dynamics by allowing access to different electronic states in the photolysis step.

Multiplex coherent anti-Stokes Raman spectroscopy study of infrared-multiphoton-excited OCS

The vibrational energy distribution following ν2 overtone excitation of OCS by a pulsed CO2 laser is studied by monitoring the coherent anti-stokes Raman spectrum of the ν1 mode. Because of the slow energy transfer from the pumped mode to other modes, and because the anharmonicity of the ν2 mode is small, OCS is an ideal system for studying the interaction of an intense infrared laser field with a single, nearly harmonic, oscillator. From the spectra the cross anharmonicities of the ν1 mode are determined to be x12=−6.0 cm−1 and x13=−2.7 cm−1, respectively. The time dependence of the spectra provides information on V–V energy transfer rates. In particular, the measurements put a lower limit of kν2→ν2=1 μs−1 Torr−1 on the vibrational relaxation rate within ν2 mode. At high excitation, the temperature of the ν2 mode rises up to 2000 K, and hot bands are observed up to the n=4 level. This fourth overtone peak is split because of either a Fermi resonance or vibrational angular momentum splitting.

Controlling the pathways in molecular decomposition: the vibrationally mediated photodissociation of water

Vibrationally mediated photodissociation, in which vibrational overtone excitation prepares a single rovibrational state of water that an ultraviolet photon subsequently dissociates, permits a fully state resolved study of the photodissociation of water. Dissociating water from six different initial rotational states and detecting the OH products by laser-induced fluorescence shows that the distribution of the products among their rotational states depends strongly on the state initially selected in the vibrational overtone excitation step. These measurements also demonstrate the control of the dissociation pathway by selection of different intermediate states. Dissociating water molecules from one vibrational state ({vert bar}04>4{sup {minus}}) produces almost no vibrationally excited OH products, but dissociating another state ({vert bar}13>{sup {minus}}) that corresponds to a different nuclear motion produces roughly comparable amounts of OH(v = 1) and OH(v = O).

Overtone excitation through fermi interaction

A new mechanism for overtone excitation processes through Fermi interaction has been proposed. The model was applied to the overtone transition of H2O from v2 = 0 to v2 = 2, and the title process was found to be dominant one. The numerical result is in good agreement with the experimental observation. Several important aspects are briefly discussed.

Vibrationally mediated photodissociation of t-butyl hydroperoxide: Vibrational overtone spectroscopy and photodissociation dynamics

Vibrationally mediated photodissociation is a two-photon technique for studying the spectroscopy and photodissociation dynamics of highly vibrationally excited molecules. In these experiments, a highly vibrationally excited t-butyl hydroperoxide (t-BuOOH) molecule, prepared by excitation in the region of the third overtone of the O–H stretching vibration (4νOH), absorbs a second photon to dissociate to OH and t-butoxy fragments, and laser induced fluorescence determines the quantum state populations of the OH fragment. Vibrational overtone excitation spectra, obtained by varying the vibrational overtone excitation wavelength while monitoring a single OH rotational state, are nearly identical to photoacoustic spectra. We fit the coarse structure in the vibrational overtone excitation spectrum in the region of the 4νOH transition and the photoacoustic spectra in the regions of the 5νOH and 6νOH transitions using a spectroscopic model of the interaction of the O–H bond stretching vibration with the torsional vibration about the O–O bond. This analysis determines the barrier to internal rotation of the O–H and t-butoxy groups through the trans configuration and its variation with vibrational excitation. The trans barrier in the ground vibrational state is 275 cm−1 and increases with vibrational excitation to 425, 575, and 680 cm−1 for t-BuOOH molecules with four, five, and six quanta of O–H stretching excitation, respectively. Comparison of the energy disposal in the vibrationally mediated photodissociation with that for direct photolysis at 376 nm, which adds the same amount of energy to the molecule, illustrates the unique dynamics that can occur when vibrational excitation precedes photodissociation. Single-photon photolysis produces fragments with large recoil velocities, while vibrationally mediated photodissociation produces slowly recoiling fragments having substantially more energy in internal excitation.

Electron-energy-loss spectroscopy of H adsorbed on Rh(100): Interpretation of overtone spectra as two-phonon bound states.

The vibrational properties of H adsorbed in the fourfold-hollow site on Rh(100) have been studied with high-resolution electron-energy-loss spectroscopy. At saturation coverage (one H per Rh atom), proper selection of the incident-electron energy and collection angle of the scattered electrons allow the detection of the symmetric (perpendicular) and asymmetric (degenerate parallel) fundamental transitions and five overtone transitions. The fundamental transitions exhibit significant frequency shifts as the H coverage is changed, establishing that H-H interactions are important in determining the local curvature of the adsorption potential-energy surface. Spectra of isotopically mixed layers (H and D) establish that significant dynamic coupling exists in both the parallel and perpendicular states at 1.0 monolayer coverage. The phonon bandwidths estimated from the isotope dilution experiments are 12--15 meV. The dynamic coupling is comparable to the local anharmonicity of the vibrational potential, and the overtones correspond to two-phonon bound-state excitations. A one-dimensional model of coupled, anharmonic oscillators is used to guide the interpretation of the overtone spectra. A qualitative interpretation of the spectra requires a proper accounting of the dispersion of both the fundamental and overtone excitations and indicates that the predominantly parallel polarized vibrations are extremely anharmonic, and the predominantly vertical polarized vibrations are only mildly anharmonic. Quantitative determination of the local anharmonicity requires a more detailed description of the H-H coupling than is currently available as the dispersion and anharmonicity are comparable.

Erratum: Dynamics of energy flow for CH overtone excitations: Theoretical and experimental studies of CH3C ≡ CH [J. Chem. Phys. 8 8, 7434 (1988)

Erratum: Dynamics of energy flow for CH overtone excitations: Theoretical and experimental studies of CH3C ≡ CH [J. Chem. Phys. <b>8</b> <b>8</b>, 7434 (1988)