Photodissociation Measurements Research Articles

State specific, angle velocity resolved measurements of photodissociation in clusters: I atom escape from (CH3I)n (n = 1,2,3)

The photodissociation of molecules in clusters affords the opportunity to study collisional relaxation of translational and rovibrational energy under solvent-like conditions. We have measured the angle velocity distribution of I( 2 P 3 2 ) and I( 2 P 1 2 ) from photodissociation of (CH 3 I) n clusters at 266 and 304 nm. For monomer ( n = 1), the angular distributions are very anisotropic and the translational energy is sharply peaked at high energy. For clusters (predominantly n = 2,3), the dissociated I atom undergoes collisions, which substantially relax the translational energy and anisotropy. Three observations are reported: (1) A residual anisotropy exists in cluster photodissociation that increases with increasing translational energy. This observation is believed to reflect the correlation of anisotropy and energy loss due to the impact parameter and number of collisions of the photofragment in the cluster. Anisotropy versus kinetic energy plots maybe interpreted to give collisional relaxation rates in clusters. (2) The I atom 2 P 3 2 to 2 P 1 2 branching ratio increases significantly in clusters relative to the monomer. Possible explanations are discussed including increased adiabaticity through the curve-crossing region in dissociation due to cage effects in analogy with reported surface results. (3) TOF spectra were observed for I atoms originating from photodissociation of the cluster photoproduct 12. The recorded spectra are consistent with an 12 energy content that is much less than the initially excited (CH 3 I) n cluster.

Photodissociation of MgC 60+ complexes generated and stored in a linear ion trap

This Letter presents a general technique for the determination of absolute photodissociation cross-sections of ionic molecular complexes in the gas phase. Experiments on MgC 60 + complexes were used to demonstrate the effectiveness of this approach. The complexes were generated collisionally inside a linear ion trap, and stored for periods of 30 s or longer during which laser induced photodissociation measurements were carried out. The dissociation rates were determined using either a mass spectrometric or a fluorescence technique, and were found to have a linear dependence on incident laser powers up to 50 mW (∼ 300 W/cm 2 intensity). The total photodissociation cross-sections increase from 10 −18 to 10 −16 cm 2 for photon energies ranging from 1.96 to 4.46 eV.

Experimental and theoretical studies of isomeric CH3S2 and CH3S+2

By combining molecular beam photodissociation and photoionization measurements with ab initio Gaussian-2 (G2) calculations on the CH3S2 and CH3S+2 systems, we have shown that CH3SS is the dominant isomer formed in the photodissociation process, CH3SSCH3+hν(193 nm) →CH3S2+CH3. The experimental ionization energy for CH3SS (8.97±0.02 eV) and the heat of formation at 0 K for CH3SS+ (217.7±1.2 kcal/mol) are in excellent agreement with the G2 results. The photoionization efficiency spectrum observed for CH3SS is also consistent with the theoretical prediction that the Franck–Condon factor for the photoionization process, CH3SS+hν →CH3SS++e−, is not favorable. Based on the statistical modeling of experimental rates obtained previously for HS loss in the unimolecular decomposition of CH3SSCH+3 and the comparison with G2 ab initio predictions, we conclude that CH2SSH+ is most likely the isomer structure formed near the experimental appearance energy (11.07 eV) observed for the photodissociative ionization process, CH3SSCH3+hν→CH3S+2+CH3+e−.

Experimental and theoretical studies of isomeric C 2H 5S and C 2H 5S +

By combining photoionization and photodissociation measurements with ab initio Gaussian-2 (G2) calculations on the C 2H 5S and C 2H 5S + system, we have concluded that CH 3CH 2S is the dominant primary product formed in the 193 nm photodissociation of (CH 3CH 2) 2S, while CH 3CHSH + is the product ion formed at the photoionization onset of C 2H 5S + from (CH 3CH 2) 2S. The G2 predictions for the heats of formation at 0 K (Δ H f0) for the isomers ofC 2H 5S and C 2H 5S +(CH 3CH 2S:Δ H f0(G2)=27.5 kcal/mol;CH 3SCH 2:Δ f0(G2)=37.3kcal/mol; CH 3CH 2S +:Δ H f0(G2)=236.5 kcal/mol;CH 3CHSH +:Δ H f0(G2)=192.6 kcal/mol; and CH 3SCH 2 +: Δ H f0(G2)=195.3 kcal/mol) are in agreement with available experimental Δ H f0 values (CH 3CH 2S: Δ H f0(exp)=31.4±2 kcal/mol; CH 3SCH 2: Δ H f0/(exp)=34.8±2.5 kcal/mol; CH 3CH 2S +: Δ H f0(exp)=238.3±2 kcal/mol; CH 3CHSH +:Δ H f0(exp)=189.6±1.0 kcal/mol;and CH 3SCH 2 +:Δ H f0(exp)=195.1 kcal/mol).The G2 calculation also yields a value of 35.3 kcal/mol for Δ H f0(CH 3CHSH).

Beyond metallo-carbohedrenes: growth and decomposition of metal-carbon nanocrystals

The recently discovered family of metal-carbon clusters, designated metallo-carbohedrenes or [open quotes]met-cars[close quotes] by Castleman and co-workers, are reinvestigated with a focus on titanium and vanadium systems. In addition to the stable M[sub 8]C[sub 12][sup +] clusters reported previously, a family of larger metal-carbon systems are observed. Mass spectroscopy and mass-selected laser photodissociation measurements are used to investigate the structures and stabilities of these systems, which contain up to 60 atoms in the cluster framework. The larger metal-carbon species exhibit 1/1 M/C stoichiometries characteristics of cubic lattice structures and are therefore designated as [open quotes]nanocrystals[close quotes]. The species Ti[sub 14]C[sub 13][sup +] and V[sub 14]C[sub 13][sup +], which are proposed to have the 3 X 3 X 3 cubic structure, are especially abundant and are believed to have stabilities comparable to those of the M[sub 8]C[sub 12] species. Photodissociation of larger nanocrystals produces both the 8/12 and 14/13 stoichiometries as abundant photofragments. Evidence is also presented for the formation of endohedral met-car clusters containing a central carbon atom, which are formed as photodissociation products from larger systems. 37 refs., 5 figs., 1 tab.

The bond energy of Co +2

The bond dissociation energy of Co + 2 has been measured by the onset of predissociation in a dense electronic spectrum by the detection of the fragment Co + ion. The value obtained, D 0 0(Co + 2) = 2.765±0.001 eV, compares well to the previously accepted value of 2.75±0.10 eV, obtained using collision-induced dissociation (CID) methods. This represents the first accurate determination of the bond strength of a transition metal cluster cation by photodissociation. The agreement between the photodissociation and CID measurements suggests that predissociation of open d-subshell transition metal clusters with a sufficient density of electronic states sets in quite abruptly at the thermodynamic threshold. Criteria for the validity of the assignment of a predissociation threshold as the thermochemical bond strength are discussed, using Co + 2 and other molecules as examples.

Dissociation energies for carbon cluster ions (C+2–15): A system where photodissociation is misleading

This letter reports a collision-induced dissociation study of the stabilities and fragmentation patterns for carbon cluster ions C+n (n = 2–15). For clusters of six or more atoms, the primary fragmentation channel is loss of C3 neutral. Fragmentation thresholds are analyzed to yield dissociation energies, which are found to be substantially higher than the upper limits estimated from photodissociation experiments of Geusic et al. [Z. Phys. D 3, 309 (1986)], but in good agreement with theoretical estimates. The stabilities oscillate strongly, with some evidence of a change in behavior as size increases above 9 atoms. This is the size where the most stable structure is believed to change from linear to cyclic. Photodissociation measurements were also done, and we find that the clusters dissociate efficiently at photon energies far below the CID thresholds. Possible reasons for the anomalous photodissociation results are discussed.

Photodissociation Spectroscopy of Gas-Phase Ferrocene Cation

The optical absorption spectrum of gas-phase ferrocene cation was measured by photodissociation (PD) spectroscopy between 570 nm and 643 nm. The PD process was loss of a cyclopentadienyl ring from the parent cation. Some structure was observed in the PD spectrum, with the highest PD being at 603 nm. The peak spacing did not correspond to a vibrational progression in the expected totally symmetric vibrational mode, and a possible assignment of the three apparent maxima involving electronic transitions from low-lying electronic states is proposed. Information on the dissociation threshold was sought by light intensity dependence measurements at 308 nm, 266 nm, and time-resolved PD measurements at 308 nm, 266 nm, and 240 nm. The PD at all of these wavelengths showed two-photon characteristics, indicating that the threshold for observable one-photon PD lies higher than about 5.4 eV.

ChemInform Abstract: Thermochemical Properties of Gas‐Phase MgOH and MgO Determined by Fourier Transform Mass Spectrometry.

AbstractThe gas‐phase values of the bond dissociation energies (D°(Mg+‐OH) = 75 kcal/mol; D°(Mg+‐O) = 53 kcal/mol) and the ionization potentials (IP(MgOH) = 7.3 eV; IP(MgO) = 7.9 eV) are determined by photodissociation measurements and charge‐transfer reactions with FT mass spectrometry.

Direct measurement of photodissociation, geminate recombination, and vibrational cooling in iodine using picosecond Raman spectroscopy

The photodissociation and geminate recombination of iodine in cyclohexane has been studied by directly monitoring the vibrational coordinates using transient Raman spectroscopy. Energy relaxation as a function of vibrational energy gap has been measured for vibrational spacings of 210 to 130 cm−1. These vibrational levels correspond approximately to v=3 to v=52 with energies 740 to 9300 cm−1 above the zero point level. The results support earlier experiments in that over 100 ps is required to completely relax the vibrational energy.

Binding energies and structures of NH 3 clusters

Optimised geometries and binding energies are presented for neutral ammonia clusters (NH 3) n with n=3 to 7. The computations treat binding potentials as pairwise additive between constituent molecules of the clusters. The relatively simple calculations give good agreement with binding energies determined by recent photoionisation and photodissociation measurements.

Charge dependence of chemisorption patterns for transition metal clusters

A method is presented for the measurement of the chemisorptive reactivity of transition metal cluster ions at near room temperature. Similar to a technique introduced previously for neutral clusters [Rev. Sci. Instrum. 56, 2123 (1985)], this cluster ion method utilizes a fast-flow reactor attached to a supersonic, laser vaporization metal cluster source, followed by time-of-flight mass spectral analysis of the cluster ions as a function of reactant concentration. Results are presented for clusters of cobalt and niobium in the 1–22 atom size range for their chemisorptive reactions with CO, CO2, and N2. Both Nb+n and Co+n clusters displayed chemical reactivity that is remarkably similar to that of the corresponding neutral clusters. For both charge states of each metal, CO was found to chemisorb with a rate which varied in a slow, monotonically increasing fashion with cluster size. The chemisorption rate of N2 and CO2, on the other hand, was found to be significantly slower than that of CO and sharply dependent upon the cluster size, this dependency being roughly independent of whether the transition metal cluster had a net positive or neutral charge. Photodissociation measurements of the mass-selected positive ion chemisorption products showed that the desorption energy of these products parallels the relative reaction rate.

Laser photodissociation of Na2+ions produced by laser resonant and non-resonant ionisation of Na vapour

The authors report direct measurements of the photodissociation of Na2+ ions created when a Na vapour in the density range 1012-1013 cm-3 is irradiated by a pulsed laser with intensities in the 105-106 W cm-2 range. The Na2+ ions, accelerated by a weak electric field, are photodissociated in a second laser beam tuned to 6040 AA. In a preliminary experiment the Na2+ ions are formed in the lowest vibrational levels and the measured photodissociation cross section is compared with existing calculations. When the Na vapour is irradiated at one of the 3S1/2-3P1/2,3/2 resonances the photodissociation measurements show the existence of a distribution of rovibrational excited levels of the Na2+ ions. A discussion is given in terms of associative ionisation in collisions of two Na(3p) atoms. The origin of the Na+ ions produced simultaneously with the Na2+ ions, in the vapour irradiated at resonance, is also discussed.

Laser photodissociation of O3+ and the energetics of ozone and its ions

Absolute cross sections for the production of O+ from O+3 have been determined at intervals of 1 nm or less through the range from 560 to 730 nm. The results are interpreted as implying an upper limit on the threshold for this process of 1.860±0.005 eV. This value, together with recent values for the adiabatic ionization potential of ozone, implies an upper limit for the bond dissociation energy of neutral ozone of 0.761±0.007 eV. This bond dissociation energy is in excellent agreement with our earlier result derived from photodissociation measurements on O3− but in serious disagreement with the literature value of 1.05±0.02 eV determined calorimetrically.

Formation of CN(B 2Σ) by the electron impact dissociation of HCN. measurements near threshold

Emission spectra of the CN violet band system (B 2Σ—X 2Σ) were observed by the electron impact on HCN with several impact energies near the threshold. The formation of CN(B) by the dissociative excitation of HCN was investigated. The threshold energy agreed essentially with that obtained by the photodissociation measurements by Okabe et al. The excitation function and the dependence of the vibrational populations of CN(B) on the electron energy were obtained. These results suggest that an optically allowed state contributes to the formation of CN(B) from HCN as a main precursor.

Absolute total cross sections for the photodissociation ofAr2+,Kr2+,Xe2+, KrN2+, and KrN+from 565 to 695 nm

Absolute measurements have been made of total cross sections for the photodissociation of $\mathrm{Ar}_{2}^{}{}_{}{}^{+}$, $\mathrm{Kr}_{2}^{}{}_{}{}^{+}$, and $\mathrm{Xe}_{2}^{}{}_{}{}^{+}$ over the photon wavelength range 565-695 nm. A drift-tube mass-spectrometer apparatus was used with a tunable dye laser for these measurements. Calculations have been made of the bound-free Franck-Condon factors between the relevant ground and dissociating states of $\mathrm{Ar}_{2}^{}{}_{}{}^{+}$ and a comparison is made to the photodissociation results. It is shown that the photodissociation measurements provide a sensitive test of ionatom potential curves and that the absolute value of the transition dipole matrix element can be obtained from a match to the experimental data. In addition, we have obtained photodissociation cross sections for Ar$\mathrm{N}_{2}^{}{}_{}{}^{+}$, Kr$\mathrm{N}_{2}^{}{}_{}{}^{+}$, and Kr${\mathrm{N}}^{+}$.

Photodissoziation bei zweiatomigen negativen Halogenmolekülionen / Photodissociation of the Diatomic Molecular Negative Ions of Halogens

Photodissociation measurements of F2 - , Cl2 -, Br2 - and I2 - in the photon energy range from 0.5 eV to 3 eV have been performed. Crossed beam technique has been applied. Absolute values from 10-18 cm2 to 10-17 cm2 are found.