Vibrational relaxation in I2−(Ar)n (n=1,2,6,9) and I2−(CO2)n (n=1,4,5) clusters excited by femtosecond stimulated emission pumping

An ab initio study on the structure, energetics and spectra of F(CO2)n−: the observation on the strong F–CO2 bond

Bulk VDE and SE values decrease from fluoride to iodide (F⁻ > Cl⁻ > Br⁻ > I⁻) in both CO₂ and H₂O.

An ab Initio Study on the Structure, Energetics, and Spectra of Cl-···(CO2)n Clusters

Structures, energetics, and photoelectron spectral properties of Cl^-···(CO₂)_n (n = 1-8) clusters are studied by ab initio electronic structure methods, namely, Møller-Plesset second-order perturbation theory (MP2) correlated consistent, aug-cc-pvtz basis functions. The most stable structure for each size is evaluated by using both bottom-up and top-down approaches. It is observed that CO₂ molecules approach to the chloride anion in an asymmetric way except for Cl^-···(CO₂)₈ cluster. We do observe the applicability of the simple classical electrostatic model for charge-quadrupole interactions to the solvation of the chloride anion by the solvent CO₂. Both vertical electron detachment energies and solvation energies are calculated for all the clusters at the MP2 level. We do observe an excellent agreement between theory and experiment for the vertical detachment energy, solvation energy, and gas phase detachment energy of chloride anion. It is also observed that the detachment energy of the chloride anion is increased by 4.09 eV due to the solvation effect of bulk CO₂ and it is quite small in contrast to the increase of 5.29 eV due to the solvation effect of polar solvent, water.

A theoretical study on structures, energetics, and spectra of Br-.nCO2 clusters: towards bridging the gap between micro-domain and macro-domain

Structures, energetics, and spectra of Br(-).nCO(2) (n = 1-8) clusters are studied based on ab initio electronic structure theory. The geometry of each size of clusters is evaluated by employing second-order Moller-Plesset (MP2) perturbation theory. It is observed that the solvent CO(2) molecules approach the bromide moiety from one side in an asymmetric fashion except for the Br(-).8CO(2) cluster. Simple electrostatic model for charge-quadrupole interactions is valid for the Br(-).nCO(2) clusters. Reduced variational space based energy decomposition method shows that the electrostatic interaction is the major component and polarization and charge transfer energies are the other significant components of the total interaction energy. Both adiabatic and vertical electron detachment energies and solvation energies are calculated at MP2 level of theory. We have observed an excellent agreement between theory and experiment for the vertical detachment and solvation energies. Calculated quantities based on the analytical expression which connects the finite domain to macroscopic one are found to be very good in agreement with the available experimental results. The present study reveals a 2.6 eV increase in the detachment energy of bromide anion due to the solvation effect of CO(2), which is relatively small compared to that of the corresponding 4.7 eV increase in detachment energy in water.

Femtosecond spectroscopy of cluster anions: insights into condensed-phase phenomena from the gas-phase

Ultrafast spectroscopy allows chemical and physical processes to be observed on time-scales faster than the nuclear motion within molecules. This tutorial review explores how such experiments, and specifically time-resolved photoelectron spectroscopy on gas-phase cluster anions, provide a molecular-level understanding of the processes that are normally associated with condensed-phase dynamics.

Ultrafast dynamics of halogens in rare gas solids

We perform time resolved pump-probe spectroscopy on small halogen molecules ClF, Cl2, Br2, and I2 embedded in rare gas solids (RGS). We find that dissociation, angular depolarization, and the decoherence of the molecule is strongly influenced by the cage structure. The well ordered crystalline environment facilitates the modelling of the experimental angular distribution of the molecular axis after the collision with the rare gas cage. The observation of many subsequent vibrational wave packet oscillations allows the construction of anharmonic potentials and indicate a long vibrational coherence time. We control the vibrational wave packet revivals, thereby gaining information about the vibrational decoherence. The coherence times are remarkable larger when compared to the liquid or high pressure gas phase. This fact is attributed to the highly symmetric molecular environment of the RGS. The decoherence and energy relaxation data agree well with a perturbative model for moderate vibrational excitation and follow a classical model in the strong excitation limit. Furthermore, a wave packet interferometry scheme is applied to deduce electronic coherence times. The positions of those cage atoms, excited by the molecular electronic transitions are modulated by long living coherent phonons of the RGS, which we can probe via the molecular charge transfer states.

Time-resolved study of solvent-induced recombination in photodissociated IBr−(CO2)n clusters

We report the time-resolved recombination of photodissociated IBr-(CO2)n (n = 5-10) clusters following excitation to the dissociative IBr-A' 2Pi12 state of the chromophore via a 180 fs, 795 nm laser pulse. Dissociation from the A' state of the bare anion results in I- and Br products. Upon solvation with CO2, the IBr- chromophore regains near-IR absorption only after recombination and vibrational relaxation on the ground electronic state. The recombination time was determined by using a delayed femtosecond probe laser, at the same wavelength as the pump, and detecting ionic photoproducts of the recombined IBr- cluster ions. In sharp contrast to previous studies involving solvated I2-, the observed recombination times for IBr-(CO2)n increase dramatically with increasing cluster size, from 12 ps for n = 5 to 900 ps for n = 8,10. The nanosecond recombination times are especially surprising in that the overall recombination probability for these cluster ions is unity. Over the range of 5-10 solvent molecules, calculations show that the solvent is very asymmetrically distributed, localized around the Br end of the IBr- chromophore. It is proposed that this asymmetric solvation delays the recombination of the dissociating IBr-, in part through a solvent-induced well in the A' state that (for n = 8,10) traps the evolving complex. Extensive electronic structure calculations and nonadiabatic molecular dynamics simulations provide a framework to understand this unexpected behavior.

Probing chemical dynamics with negative ions

Experiments are reviewed in which key problems in chemical dynamics are probed by experiments based on photodetachment and/or photoexcitation of negative ions. Examples include transition state spectroscopy of biomolecular reactions, spectroscopy of open shell van der Waals complexes, photodissociation of free radicals, and time-resolved dynamics in clusters. The experimental methods used in these investigations are described along with representative systems that have been studied.

Rydberg states of small NaArn* clusters

The 4s and 5s Rydberg excited states of NaAr(n)* clusters are investigated using a pseudopotential quantum-classical method. While NaAr(n) clusters in their ground state are known to be weakly bound van der Waals complexes with Na lying at the surface of the argon cluster, isomers in 4s or 5s electronically excited states of small NaAr(n)* clusters (n< or =10) are found to be stable versus dissociation. The relationship between electronic excitation and cluster geometry is analyzed as a function of cluster size. For both 4s and 5s states, the stable exciplex isomers essentially appear as sodium-centered structures with similar topologies, converging towards those of the related NaAr(n)+ positive ions when the excitation level is increased. This is consistent with a Rydberg-type picture for the electronically excited cluster, described by a central sodium ion solvated by an argon shell, and an outer diffuse electron orbiting around this NaAr(n)+ cluster core.

Vibrational Relaxation of Molecular Ions at Low Temperatures: O2-(v=1) + Ar

The vibrational relaxation of oxygen molecular ions trapped in an argon cage in the temperature range 10-85 K has been studied using semiclassical procedures. The collision model is based on the trapped molecule undergoing the restricted motions (local translation and hindered rotation) in a cage formed by its 12 nearest argon neighbors in a face-centered cubic arrangement. At 85 K in the liquid argon temperature range, the relaxation rate constant of O2(-) (v=1) is 1130 s(-1). The rate constant decreases to 270 s(-1) at 50 K and to 3.90 s(-1) at 10 K in the solid argon temperature range. In the range 10-85 K, the rate constant closely follows the temperature dependence k proportional to T2.7. Energy transfer pathways for the trapped molecular ion are vibration to local translation, argon phonon modes, and rotation (both hindered and free).

Photodissociation dynamics of IBr−(CO2)n, n<15

We report the ionic photoproducts produced following photoexcitation of mass selected IBr(-)(CO(2))(n), n=0-14, cluster ions at 790 and 355 nm. These wavelengths provide single state excitation to two dissociative states, corresponding to the A(') (2)Pi(1/2) and B 2 (2)Sigma(1/2) (+) states of the IBr(-) chromophore. Excitation of these states in IBr(-) leads to production of I(-)+Br and Br(-)+I( *), respectively. Potential energy curves for the six lowest electronic states of IBr(-) are calculated, together with structures for IBr(-)(CO(2))(n), n=1-14. Translational energy release measurements on photodissociated IBr(-) determine the I-Br(-) bond strength to be 1.10+/-0.04 eV; related measurements characterize the A(') (2)Pi(1/2)<--X (2)Sigma(1/2) (+) absorption band. Photodissociation product distributions are measured as a function of cluster size following excitation to the A(') (2)Pi(1/2) and B 2 (2)Sigma(1/2) (+) states. The solvent is shown to drive processes such as spin-orbit relaxation, charge transfer, recombination, and vibrational relaxation on the ground electronic state. Following excitation to the A(') (2)Pi(1/2) electronic state, IBr(-)(CO(2))(n) exhibits size-dependent cage fractions remarkably similar to those observed for I(2) (-)(CO(2))(n). In contrast, excitation to the B 2 (2)Sigma(1/2) (+) state shows extensive trapping in excited states that dominates the recombination behavior for all cluster sizes we investigated. Finally, a pump-probe experiment on IBr(-)(CO(2))(8) determines the time required for recombination on the ground state following excitation to the A(') state. While the photofragmentation experiments establish 100% recombination in the ground electronic state for this and larger IBr(-) cluster ions, the time required for recombination is found to be approximately 5 ns, some three orders of magnitude longer than observed for the analogous I(2) (-) cluster ion. Comparisons are made with similar experiments carried out on I(2) (-)(CO(2))(n) and ICl(-)(CO(2))(n) cluster ions.

Time-resolved intraband electronic relaxation dynamics of Hgn− clusters (n=7–13,15,18) excited at 1.0 eV

Time-resolved photoelectron imaging has been used to study the relaxation dynamics of small Hg(n) (-) clusters (n=7-13,15,18) following intraband electronic excitation at 1250 nm (1.0 eV). This study furthers our previous investigation of single electron, intraband relaxation dynamics in Hg(n) (-) clusters at 790 nm by exploring the dynamics of smaller clusters (n=7-10), as well as those of larger clusters (n=11-13,15,18) at a lower excitation energy. We measure relaxation time scales of 2-9 ps, two to three times faster than seen previously after 790 nm excitation of Hg(n) (-), n=11-18. These results, along with size-dependent trends in the absorption cross-section and photoelectron angular distribution anisotropy, suggest significant evolution of the cluster anion electronic structure in the size range studied here. Furthermore, the smallest clusters studied here exhibit 35-45 cm(-1) oscillations in pump-probe signal at earliest temporal delays that are interpreted as early coherent nuclear motion on the excited potential energy surfaces of these clusters. Evidence for evaporation of one or two Hg atoms is seen on a time scale of tens of picoseconds.

The influence of cluster formation on the photodissociation of sulfur dioxide: Excitation to the E state

A femtosecond pump-probe technique was employed to study the dissociation dynamics of sulfur dioxide and sulfur dioxide clusters in real time. Dissociation is initiated by a multiphoton scheme that populates the E state. The SO(2) (+) transient is fit to a biexponential decay comprising a fast and a slow component of 230 fs and 8 ps, respectively. The SO(+) transient consists of a growth component of 225 fs as well as a subsequent decay of 373 fs. The pump-probe response obtained from the monomer clearly shows the predissociative cleavage of a S-O bond. Upon cluster formation, a sequential increase in the fast decay component is observed for increasing cluster size, extending to 435 fs for (SO(2))(4) (+). The transient response of cluster dissociation products SO(SO(2))(n) (+), where n=1-3, reflects no growth component indicating that formation proceeds through the ion state. Therefore, cluster formation results in a caging effect, which impedes the dissociation process. Further direct evidence for our proposed mechanism is obtained by a technique that employs a comparison of the amplitude coefficients of each respective component of the fit. This method makes possible the determination of branching ratios of competing relaxation processes and thereby the influence of cluster formation on each can be resolved. The caging effect is attributed to a steric hindrance placed on the SO(2) chromophore, preventing it from attaining a linear geometry necessary for dissociation.

Replacement equivalence of H- and argon in small (Ar)nH- clusters from optimized structure calculations

The structural properties of some of the smaller ionic clusters of argon atoms containing the atomic impurity H-, ArnH- with n from 2 up to 7, are examined using different modeling for the interactions within each cluster and by employing different theoretical treatments, both classical and quantum, for the energetics. The same calculations are also carried out for the corresponding neutral homogeneous clusters Ar(n+1). The results of the calculations, the physical reliability of the interactions modeling, and the similarities and the difference between the anionic and the neutral complexes are discussed in some detail. The emerging picture shows that, due to specific features of the employed atom-atom potentials, the ArnH- and Ar(n+1) clusters present very similar structures, where the H- dopant substitutes for one of the outer Ar atoms but does not undergo as yet solvation within such small clusters.

Vibrational relaxation of CH3I in the gas phase and in solution

Transient electronic absorption measurements reveal the vibrational relaxation dynamics of CH(3)I following excitation of the C-H stretch overtone in the gas phase and in liquid solutions. The isolated molecule relaxes through two stages of intramolecular vibrational relaxation (IVR), a fast component that occurs in a few picoseconds and a slow component that takes place in about 400 ps. In contrast, a single 5-7 ps component of IVR precedes intermolecular energy transfer (IET) to the solvent, which dissipates energy from the molecule in 50 ps, 44 ps, and 16 ps for 1 M solutions of CH(3)I in CCl(4), CDCl(3), and (CD(3))(2)CO, respectively. The vibrational state structure suggests a model for the relaxation dynamics in which a fast component of IVR populates the states that are most strongly coupled to the initially excited C-H stretch overtone, regardless of the environment, and the remaining, weakly coupled states result in a secondary relaxation only in the absence of IET.