Femtosecond study of Xe-Cl2 reactions in clusters

100

126

Two-dimensional spectroscopy: Theory of nonstationary, time-dependent absorption and its application to femtosecond processes J. Chem. Phys. 91, 4485 (1989); 10.1063/1.456786 Femtosecond resolution ultrafast ion reaction time measurement a pulsed-laser field desorption time-of-flight spectroscopy AIP Conf. Proc. 146, 610 (1986) This paper in the series gives our full account of the preliminary results reported in a communication ͓Cheng, Zhong, and Zewail, J. Chem. Phys. 103, 5153 ͑1995͔͒ on real-time femtosecond ͑fs͒ studies of the transition state of charge-transfer ͑CT͒ reactions, generally described as harpooning reactions. Here, in a series of experimental studies in a molecular beam, and with the help of molecular dynamics, we elucidate the microscopic elementary dynamics and the structure of the transition states for the isolated, bimolecular reaction of benzenes ͑electron donor͒ with iodine ͑electron acceptor͒. The transition state is directly reached by fs excitation into the CT state of the complex Bz•I 2 , and the dynamics is followed by monitoring the product build up or the initial transition-state decay. We further employed the fs resolution in combination with the kinetic-energy resolved time-of-flight and recoil anisotropy techniques to separate different reaction pathways and to determine the impact geometry. Specifically, we have studied: ͑1͒ the temporal evolution of the transition state ͑ ‡ ͒ and of the final products ͑ ͒; ͑2͒ the product translational-energy distributions; ͑3͒ the recoil anisotropy ͑␤͒ in each channel; ͑4͒ the reaction time dependence on the total energy; ͑5͒ the dynamical and structural changes with varying CT energy ͑ionization potential-electron affinity-Coulomb energy͒. Such a change is made by replacing the electron donor from benzene to toluene, and to xylenes and trimethylbenzenes of different symmetries. We have also studied deutrobenzene as a donor. The reaction mechanism involves two exit channels. The first one ͑ionic͒ follows the ionic potential of the CT state. Following the harpooning ͑Bz ϩ•I 2 Ϫ ͒, the transition state•I͔* ‡ evolves on the adiabatic potential to produce Bz ϩ •I Ϫ and I products. The second channel ͑neutral͒ is due to the coupling of the transition state to neutral, locally excited, iodine repulsive states and, in this case, the products are Bz•IϩI. The latter process is an intermolecular electron transfer and occurs on an ultrafast time scale of 250 fs, resulting in a greater yield for the neutral channel. Molecular dynamics simulations support this dynamical picture and provide the time scales for trajectories in the transition-state region and in the product valley. The geometry of the transition state is determined from the anisotropy measurements and we found a nearly axial geometry with the iodine axis of recoil tilted 30°-35°away from the transition moment. These angular dependencies are related to the molecular structure and the electronic structure with highest occupied molecular orbit-lowest occupied molecular orbit descriptions. By...

Section: Introductionmentioning

confidence: 99%

Femtosecond real-time probing of reactions. XXI. Direct observation of transition-state dynamics and structure in charge-transfer reactions

Cheng

Zhong

Zewail

1996

100

126

“…8 If the lifetime of this intermediate complex is short, however, the initial conditions still determine the reaction outcome. 31 Recent two-photon femtosecond studies 17,32 suggest that this is in fact the case in the reaction originating from collision pairs in low pressure gas mixtures. In the latter study, 32 Dedonder-Lardeux et al also identified a second entrance channel occurring some 500 fs after the initial photon absorption in the reaction originating from vdW complexes.…”

Section: Discussionmentioning

confidence: 99%

“…31 Recent two-photon femtosecond studies 17,32 suggest that this is in fact the case in the reaction originating from collision pairs in low pressure gas mixtures. In the latter study, 32 Dedonder-Lardeux et al also identified a second entrance channel occurring some 500 fs after the initial photon absorption in the reaction originating from vdW complexes. This delayed entrance channel is also transient, and therefore is thought to occur within Xe n Cl 2 clusters where cage effects allow a recoil of the dissociating Cl atom.…”

Section: Discussionmentioning

confidence: 99%

Energy disposal in the two-photon laser-assisted reaction in xenon and chlorine gas mixtures

Kohel

Keto

2000

The two-photon laser assisted reaction (LAR) in low pressure xenon and chlorine gas mixtures has been studied over a broad range of excitation wavelengths in order to characterize the role of the entrance channel in determining the vibrational state distribution of the reaction products. We measure a high degree of vibrational excitation in the XeCl* product, confirming observations from previous studies of the LAR of Xe+Cl2 collision pairs [Ku et al., J. Phys. Chem. 87, 2989 (1983)], and in distinct contrast with the vibrationally cold excimer observed following laser excitation in molecular beam experiments [Boivineau et al., Chem. Phys. Lett. 128, 528 (1986)]. The mean vibrational energy in the XeCl excimer depends strongly on laser wavelength, increasing with decreasing wavelength. Moreover, an increasing fraction of the available energy from the reaction is observed as vibration in the XeCl* product as the laser is tuned toward shorter wavelengths. The reaction outcomes are interpreted in terms of a selectivity of initial conditions on the ionic potential surface that mediates the reactive collision.

“…Laser induced harpoon reactions, the most common of which are those between rare gas atoms ͑Rg͒ and molecular halogens ͑X 2 ͒ or halogen containing polyatomic molecules ͑RX͒, have been extensively studied in the gas phase, [1][2][3][4][5][6][7][8] in clusters produced in molecular beams, [9][10][11] in the solid state, [12][13][14][15][16] and in the liquid phase. 13,14,[17][18][19] This class of photochemical reactions is useful in studies of fundamental chemical dynamics, and in particular, bimolecular chemistry, in condensed media.…”

Section: Introductionmentioning

confidence: 99%

“…19 This observation has been more recently verified by time resolved measurements in clusters containing Cl 2 and Xe. 10 In the latter two color experiments, with a probe wavelength of 308 nm, a delayed transient signal due to the sequential process of Eq. ͑5͒ could also be seen.…”

Section: Introductionmentioning

confidence: 99%

Photodynamics of charge transfer and ion-pair states of Cl2:Xe complexes in liquid Ar

Hill

Apkarian

1996

Sequential two photon studies of Xe:Cl 2 complexes in liquid Ar are reported. The first photon prepares the excited state complex Xe:Cl 2 ͑ 3 ⌸ u ͒, which radiatively relaxes with a lifetime of 10Ϯ1 s ͑22Ϯ2 s for the uncomplexed Cl 2 ͑ 3 ⌸ u ͒͒. A second photon accesses either the molecular ion pair state, Xe:Cl ϩ Cl Ϫ , or the contact charge transfer transition, Xe ϩ Cl 2 Ϫ. The latter dissociates to Xe ϩ Cl Ϫ ϩCl even though the product is formed at threshold. The former undergoes charge rearrangement and decays radiatively via Xe ϩ Cl 2 Ϫ ͑ 2 ⌺ u ͒→XeCl 2 ͑ 1 ⌺ g ͒. Analysis of the excitation and emission spectra of the key intermediate in laser induced harpoon reactions, Xe ϩ Cl 2 Ϫ , leads to the conclusions that the complex is linear in both ionic and neutral states, and allows a direct contrast between dissociation dynamics on ionic versus neutral surfaces.