Vibrational Energy Content Research Articles

Vibrational relaxation in clusters: Energy transfer in I2−(CO2)4 excited by femtosecond stimulated emission pumping

Vibrational relaxation dynamics in I2−(CO2)4 clusters are monitored by femtosecond stimulated emission pumping in conjunction with femtosecond photoelectron spectroscopy. Femtosecond pump and tunable dump pulses coherently excite the I2− within the cluster with vibrational energies ranging from 0.57 to 0.86 eV; the subsequent dynamics are monitored via the time-dependent photoelectron spectrum, and are compared to those resulting from excitation of bare I2−. Two observables are used to follow the vibrational relaxation from the vibrationally excited I2− to the surrounding solvent molecules. From 0 to 4 ps, relaxation is apparent through a time-dependent increase in the oscillation which is monitored at its inner turning point. At longer times, out to ∼100 ps, shifts in the photoelectron spectra are used to determine the vibrational energy content of the I2−. Indirect evidence is presented for early rapid energy loss during the first half-oscillation of the wave packet across the potential.

Collision-induced intramolecular energy flow and C–H bond dissociation in excited toluene

The collision-induced intramolecular energy flow and C–H bond dissociation in toluene have been studied using classical dynamics procedures. The molecule initially contains high amounts of vibrational excitation in the methyl C–H stretch and the nearby benzene ring C–H stretch and it is in interaction with Ar. The two excited C–H stretches are coupled to each other through two C–C stretching, two H–C–C bending and one C–C–C bending modes, all of which are initially in the ground state. At 300 K, the energy lost by the excited molecule upon collision is not large and it increases slowly with increasing total vibrational energy content between 10 000 and 40 000 cm−1. Above the energy content of 40 000 cm−1, energy loss increases rapidly. Near 65 000 cm−1 energy loss takes a maximum value of about 1000 cm−1. The temperature dependence of energy loss is weak between 200 and 400 K. When the energy content is sufficiently high, either or both C–H bonds can dissociate, producing free radicals, C6H5CH2, C6H4CH3, or C6H4CH2. The ring C–H dissociation occurs almost entirely in a direct-mode mechanism on a subpicosecond time scale. Nearly half of methyl group C–H dissociation events occur on a subpicosecond time scale and the rest through a complex-mode collision in which bond dissociation occurs several picoseconds after the initial impact. In the complex-mode collision, Ar binds to the radical forming a weakly bound benzyl⋯Ar complex. In both dissociative and nondissociative events, intramolecular energy flow is efficient, taking place upon the initial impact on a subpicosecond time scale.

The production and decay kinetics of ClOO in water and freon-11: A time-resolved resonance raman study

The production of ClOO following OClO photolysis in water and fluorotrichloromethane (freon-11) is investigated using time-resolved resonance Raman (TRRR) spectroscopy. Stokes spectra are obtained as a function of time following OClO photoexcitation using pump and probe wavelengths of 390 and 260 nm, respectively. Scattering assignable to ClOO is observed, and appears with a time constant of 27.9±4.5 ps in water and 172±30 ps in freon-11. The ClOO intensity decays with a time constant of ∼398±50 ps in water and 864±200 ps in freon-11. Although the production and decay kinetics are solvent dependent, the quantum yield for ClOO production is similar between water and freon-11. Femtosecond pump–probe studies designed to monitor the evolution in optical density at 390 and 260 nm following OClO photoexcitation are also presented. These studies demonstrate that geminate recombination of the primary photoproducts is less efficient in freon-11 relative to water. This result taken in combination with the solvent invariance of the ClOO-production quantum yield indicates that ClOO is not formed via geminate recombination. Instead, the results presented here suggest that OClO photoisomerization results in the production of ClOO. Finally, the vibrational energy content of ClOO upon internal conversion to the ground state is studied through comparison of the ClOO Raman and absorption cross sections to those predicted using computational methods. These studies suggest that ground-state ClOO is produced with minimal excess vibrational energy. The results presented here provide new insight into the mechanism of ClOO formation following OClO photoexcitation.

Photodissociation of NeBr2(B) below and above the dissociation limit of Br2(B)

The photodissociation dynamics of the NeBr2 complex in the B electronic state is studied, for the first time, near the Br2(B) dissociation limit, below and above, when the complex is promoted from the ground T-shaped level in the X electronic state. A time-dependent treatment is used in which the initial wave packet is divided in two portions, one describing the slow predissociation dynamics below the Br2 dissociation threshold, and the second one, the fast complete dissociation in Ne+Br+Br fragments. Below that threshold, the absorption spectrum shows an increasing congestion as the vibrational energy content of Br2 increases, but narrow peaks appear again for the highest energy region of the spectrum. These peaks correspond to long lived resonances associated with “horseshoe” type states, as demonstrated by two-dimensional calculations. These resonances have a significant probability density for the linear geometry in which the Ne atom is inserted between the two bromine atoms. At this configuration the exchange of vibrational energy is rather inefficient which explains both why the spectrum is so sparse and resonances are so narrow. Above the Br2 dissociation threshold, the recombination of Br2 is found to be very inefficient, except for very low kinetic energies. The small recombination probabilities are due to vibrational couplings and not to any collisional caging effect. Since the complex remains essentially T-shaped during dissociation, extensive two-dimensional calculations are performed for longer times to better determine final vibrational distributions at low kinetic energies.

Anharmonic Quasiclassical Barrier Samplings in Trajectory Calculations and Their Influence on the Computed Product Energy Distributions

Harmonic and anharmonic quasiclassical barrier samplings were used in classical dynamics simulations to evaluate product energy distributions (PEDs). The results obtained for the CH dissociation in the methanethiol cation (CH3SH+ → CH2SH+ + H) show that the PEDs are changed under inclusion of anharmonicity in the initial conditions. Both the vibrational energy content at the transition state and the energy in the transitional modes are important to explain the differences found in the PEDs. Discrepancies between the PEDs obtained for trajectories initiated at the barrier and those initiated at the reactant were found and explained on the basis of dynamical phase space bottlenecks, which make the phase space density not uniform for t ≠ 0.

Open-system quantum dynamics for laser-induced DIET and DIMET

A time-dependent open-system density matrix approach to the UV/visible-laser-induced, “hot-electron”-mediated photodesorption of small neutral molecules from metal substrates is reviewed, and a few new applications are added. Both the single excitation-de-excitation (DIET, desorption induced by electronic transitions) and multiple excitation-de-excitation (DIMET, desorption induced by multiple electronic transitions) limits are considered. The theoretical framework is presented and applied to DIET of NO from Pt(111) and NH 3 from Cu(111), as well as to DIMET of NO from Pt(111). Special emphasis is given (i) to the estimate of excited state lifetimes, (ii) to the translational and vibrational energy content of the desorbates, (iii) to the scaling of photodesorption yields and other properties with laser fluence in the DIMET case, and (iv) to the possibility of controlling photochemistry at surfaces despite strong electronic relaxation.

Kinetics of fullerene triplet states

Studies are described whose goal is a quantitative kinetic description of fullerene triplet relaxation. The room-temperature intrinsic lifetimes of solution phase T1 C60 and C70 differ substantially, with values in toluene of 143 μs and 12 ms, respectively. These decay rates exhibit only weak temperature dependence near room temperature. The intrinsic lifetime of T1 C60 has a simple dependence on vibrational energy content up to 1000 K. Efficient triplet-triplet annihilation occurs in C60 and C70 solutions at ca. 50% of the diffusion-limited rate. In mixed solutions, rapid reversible triplet energy exchange was observed between C60 and C70, and between C60 and (CH3)2 C60. A new method for measuring relative triplet enthalpies and entropies in such mixtures has also been applied. Complex kinetics has been uncovered in C70 solutions and modeled by reversible formation of shortlived triplet excimers, accounting for the efficient self-quenching. C60 self-quenching has been found to be highly temperature dependent, but the mechanism remains unresolved.

Diode laser measurements of CD3 quantum yields and internal energy for the dissociation of dimethyl sulfoxide-d6

Infrared diode laser absorption spectroscopy has been used to measure the CD3 radical photoproducts formed in the 193 and 222 nm photodissociation of dimethyl sulfoxide-d6. Quantum yields of CD3 have been determined to be 1.4±0.1 at 193 nm and 1.2±0.2 at 222 nm, compared to 2.0 for acetone-d6 at 193 nm. An analysis of transient waveforms reflecting the vibrational relaxation and radical recombination kinetics also yields an estimate of the nascent CD3 vibrational energy content by determining the fraction of total CD3 initially produced in the vibrationless state. The nascent CD3 population in the vibrational ground state decreases in order for the following photodissociation systems: CD3I at 248 nm, DMSO-d6 at 193 and 222 nm and acetone-d6 at 193 nm. The DMSO results are in good agreement with recent photofragment translational spectroscopy results and support a stepwise mechanism for the dissociation of DMSO at both wavelengths.

Ultrafast Study of the Photodissociation and Recombination of Aqueous O3-

The photodissociation of the strongly solvated radical O3- (generating O2 and O-) has been studied in aqueous solution by femtosecond pump−probe spectroscopy. The near-UV absorption band of O3- exhibits a prompt bleach due to photolysis (390 nm excitation), which is followed by a partial recovery (3.5 ps) that is assigned to delayed geminate recombination. The transient spectrum of O3- shows no evidence of excess vibrational energy content, indicating that vibrational relaxation of O3- occurs on a faster time scale than the recombination process (3.5 ps). This overall picture is in strong contrast to the standard prototype for solution recombination reactions, i.e. the widely investigated photodissociation of I2 in nonpolar solvents. For I2, photodissociation and prompt recombination (∼0.3 ps) lead to hot I2 molecules that relax on the tens of picoseconds time scale. The extremely fast vibrational relaxation and the slow recombination of O3- in water apparently stem from strong solute−solvent interactions for both O3- and O-. The O3- behavior may in fact be a common situation for polar reactions in polar solvents. In these studies, O3- was generated via a known reaction by photolysis of an oxygenated basic solution of hydrogen peroxide. The quantum yield of cage escape for O3- photodissociation has been measured to be 0.50 ± 0.02, assuming that the long-lived bleach is due entirely to cage escape. The long-lived bleach may, however, also be due to other photophysical pathways that involve long-lived, low-lying O3- excited states. Both the apparent cage escape yield and recombination dynamics are identical within experimental error in H2O and D2O. The rotational reorientation time of O3- has been measured to be 2.3 ps.

Impact-induced dissociation of methane and ethane on Pt(111) and Pt0.25Rh0.75(111)

The results of supersonic molecular beam studies of the sticking of methane and ethane on Pt(111) and Pt0.25Rh0.75(111) substrates as a function of the translational and vibrational energy content of the incident molecules are described. Simple analyses of these experiments indicate that, in both methane and ethane, sticking is particularly enhanced for molecules with two quanta of vibrational excitation in C—H deformation modes. The results of this analysis have been employed in a simulation of the thermal sticking of ethane on platinum.

Quantum-state-specific metastable time-of-flight as a probe of CO photodesorption from GaAs(100)

By exciting the ‘forbidden’ a 3 Π ← X 1 Σ + bands of CO with an intense ultraviolet laser pulse, neutral time-of-flight (TOF) distributions of single quantum states of CO can be obtained using long (> 10 cm) flight paths. This new method is applied here to surface photodesorption in order to simultaneously determine the rotational, vibrational and translational energy content of CO formed in the reaction CO/GaAs(100) + hv(248 nm) → CO(X 1 Σ +, v ⩽ 4, J ⩽ 24) + GaAs(100). Distinctly non-Boltzmanntranslational energy distributions were obtained for every observed quantum state of CO. Other state specific observations are discussed.

The initial vibrational state distribution of HCN X̃ 1Σ+(v1,0,v3) from the reaction CN(2Σ+)+C2H6→HCN+C2H5

The reaction of the cyano radical (CN) with ethane was studied using time-resolved infrared absorption spectroscopy to monitor individual rovibrational states of the HCN product. A method is described that can be used to determine the initial vibrational state distribution at pressures of several Torr. This technique was applied to the title reaction to determine that the vibrational states of HCN(v1,0,v3), where v1, v3=0, 1, and 2, were not directly populated in the title reaction to any significant extent. The initial vibrational energy content of the CN radical was also varied but did not influence the initial population in the HCN vibrational levels probed in this experiment. The time dependence of HCN(v1,0,v3) was followed and interpreted in terms of bimolecular rate constants for vibrational relaxation with ethane. The title reaction is mode specific in its energy disposal in that at least every HCN product appears to have at least one quantum of bending excitation, likely in combination with stretching vibrations.

Subpicosecond vibrational relaxation of the S1 states of azulene and guaiazulene in solution

The S1 population decay times of azulene and 1,4-dimethyl-7-isopropylazulene (guaiazulene) in solution have been determined as a function of their initial vibrational energy content, [Formula: see text] using a pump–probe experiment with subpicosecond time resolution. The S1 lifetime of azulene does not depend on [Formula: see text] for energies up to 1760 cm−1, whereas the lifetime of the shorter-lived S1 state of guaiazulene is independent of [Formula: see text] only for energies up to ~1000 cm−1. At higher energies, the lifetime decreases with increasing [Formula: see text] and exhibits the same behavior in two structurally different solvents. It is suggested that internal conversion to the electronic ground state from a vibrationally unrelaxed S1 state is responsible for the effects observed in guaiazulene, and that intramolecular vibrational redistribution occurs with a time constant of several hundred femtoseconds. Keywords: pump–probe spectroscopy, azulene, femtosecond processes.

Rotational energy transfer in highly vibrationally excited HCN

The state-resolved collisional self-relaxation of HCN at a vibrational energy content of 10 000 cm−1 is probed directly by combining direct overtone vibration excitation, to prepare energized molecules in the (0,00,3) level, with a laser induced fluorescence monitor of the population evolution from different rotational states. Pure rotational energy transfer dominates the collision dynamics while vibrational relaxation results from only a small fraction of the inelastic events. The depopulation of single j levels proceeds with high efficiency. It is characterized by rates up to 14 times faster than the Lennard-Jones gas kinetic rate conforming to a j dependent distribution which peaks near the Boltzman population maximum and decreases to higher and lower angular momentum values. Approximately 70% of the collisional population removal from the j=4 level proceeds via the ΔJ=±1 channel and 28% proceeds via the ΔJ=±2 direct population transfer step. The results support a long range dipole–dipole mechanism for the energy transfer. This work also investigated various empirical scaling relations and determined that a two parameter fitting law based on the momentum gap or a three parameter modified scaling expression based on the energy gap successfully models the rotational relaxation.

On the vibrational temperature of metal cluster beams: A time-resolved thermionic emission study

Delayed ionization rates for small niobium clusters are measured as a function of the cluster size, the laser wavelength, the number of photons absorbed, and the initial internal energy content of the cluster. It is shown, for the first time, that vibrational excitation of the clusters modulates their rate of delayed ionization. An analysis of the rate of ionization in terms of the total energy content of the cluster establishes unequivocally that delayed ionization is a statistically determined, unimolecular, activated process. It is shown that the rate of delayed ionization can be used to gauge the initial vibrational energy content of the cluster. Quantitative analysis of this effect establishes, for the first time, a thermometry for metal cluster beams. Thermal rate parameters, activation energies, and ‘‘Arrhenius factors’’ are presented for delayed ionization of a series of niobium clusters Nbn, n=5, 6, 7, 8, 9, 11, 12, and 13. The activation energies are lower than the corresponding ionization potentials by ∼1 eV. The ‘‘Arrhenius factors’’ are all in the range 1011 s−1. These parameters highlight the differences between delayed ionization and photoionization of clusters and draw attention to the need for an adequate theory of unimolecular processes in clusters taking into account both nonadiabatic effects and the important effects arising from the fluxionality of the cluster at high internal energies.

Collisional vibrational relaxation of a triplet state: Energy-dependent energy loss from T1 pyrazine

The loss of vibrational energy from gas phase T1 pyrazine molecules has been measured for thermal collisions with helium, argon, H2, SF6, and ground state pyrazine. Triplet pyrazine was prepared with a well defined vibrational energy of 5433 cm−1 through S1→T1 intersystem crossing following optical excitation to the 8a1 level of S1. The time-dependent vibrational energy content of the excited pyrazine molecules was then deduced using a recently developed ‘‘direct’’ method involving the kinetics of subsequent T1→S0 intersystem crossing. For each of the collision partners studied, it was possible to find the average energy lost per gas kinetic collision for donor energies ranging from ca. 2000 to 5433 cm−1. The magnitudes of these energy losses generally increased with the mass and vibrational complexity of the relaxing collision partner. For vibrational energy contents near 5000 cm−1, relaxation of the triplet pyrazine was enhanced by factors of as much as 24 relative to S0 benzene at a similar vibrational energy. In addition, with all collision partners studied the average energy lost per collision showed appparent threshold behavior near 3000 cm−1, increasing by approximately an order of magnitude as the donor’s energy increased from 2500 to 5000 cm−1. The findings of this first quantitative study of triplet relaxation suggest that collisional vibrational energy transfer from organic triplet states may proceed by mechanisms different from those that dominate ground state relaxation.

Doppler spectroscopy of the OH fragment ejected by t r a n s HONO (à 1A″): Characterization of the à state resonances and determination of vibrational energy content of the NO fragment

Resonances in the quasistable à 1A″ state of trans HONO are probed through photodissociation experiments. The OH fragment ejected by HONO excited to specific –N■O stretching vibrational levels of the à state is probed by high-resolution sub-Doppler spectroscopy of the R2(1) (1,0) A 2Σ+–X 2Π transition. The Doppler profiles provide information on the vibrational state dependence of HONO’s subpicosecond fragmentation time and OH fragment’s kinetic energy. The latter is used to deduce NO sister fragment’s translational and vibrational energy content. These provide quantitative probes for (a) V→T ‘‘energy transfer’’ from the photoselected N■O vibrational coordinate to the recoil O–N coordinate and (b) N■O ‘‘vibrational relaxation’’ during dissociation. The present results and our previous measurements on the HONO ÖX̃ linewidths are discussed in terms of an excited-state potential energy surface in which (a) the –N■O coordinate is coupled to the reaction coordinate (O–N) and (b) there is a bottleneck in the exit channel.

Dynamics of the thermal decomposition of 2,3-diazabicyclo[2.2.1]hept-2-ene

The dynamics of the thermal decomposition of 2,3-diazabicyclo [2.2.1] hept-2-ene (DBH) have been investigated over the temperature range 900-1400 K. A tunable continuous wave CO laser was used to follow the vibrational energy content of the coupled CO−N 2 system from the initial to the equilibrium conditions. It has been shown that the N 2 is born with very little vibrational energy, while the bicyclo [2.1.0] pentane is born with an excess over the equilibrium distribution. These results provide clear evidence against the concerted decomposition mechanism

Born–Oppenheimer dynamics using density-functional theory: Equilibrium and fragmentation of small sodium clusters

The properties of small neutral and positively charged sodium clusters and the fragmentation dynamics of Na++4 are investigated using a simulation technique which combines classical molecular dynamics on the electronic Born–Oppenheimer ground-state potential surface with electronic structure calculations via the local spin-density functional method. Results for the optimal energies and structures of Nan and Na+n (n≤4) are in quantitative agreement with previous studies and experimental data. Fission of Na++4 on its ground state Born–Oppenheimer potential-energy surface, following sudden ionization of selected configurations of an Na+4 (or Na4) cluster, whose vibrational energy content corresponds to 300 K, is found to occur on a picosecond time scale. The preferred fission channel is found to be Na+3+Na+, with an interfragment relative translational kinetic energy of ∼2 eV, and a vibrationally excited Na+3. The dynamics of the fragmentation process is analyzed.

Chemical effects of radical vibrational energy content on the abstraction reaction: CF3 + Br2 → CF3Br + Br

AbstractWe have studied the effect of vibrational mode activation in the CF3 radical on the bromine abstraction reaction; CF3 + Br2 → CF3Br + Br. Excess vibrational energy resides in the symmetric modes of the radical after 248 nm photolysis of the parent molecule, CF3I. Our data indicate that the hot radicals react no faster than thermalized CF3, and may actually have a lower cross‐section for reaction. Dynamical factors that result in poor coupling of the vibrational energy to the reaction coordinate, as well as other similar considerations, could be responsible for the experimental observations. In addition, we have made an independent determination of the rate for the bromine abstraction reaction of (1.08 ± .13) × 1012 s−1 cm3 mol−1.