C. A. Woodward

The photofragmentation of Ar+3

The photofragment kinetic energy spectrum of Ar+3 has been recorded in a crossed‐beam apparatus at 532 nm; the only photofragment product observed is Ar+. For the purposes of analysis, a spectrum for the photofragmentation of Ar+2 was recorded under similar experimental conditions. In each case, the ions were prepared by the electron impact ionization of a neutral argon cluster beam. The Ar+3 spectrum consists of two quite distinct features, a high‐energy component which closely resembles the result observed for Ar+2, and a second, low‐energy feature, which is peculiar to Ar+3 alone. The two high‐energy wings appear to arise from a very rapid dissociation process where approximately 70% of the excess energy appears as Ar+ kinetic energy. A computer simulation of this region of the spectrum gives an anisotropy parameter, β, of 1.1±0.2. The low‐energy, component to the spectrum arises from a two‐step dissociation process, in which a weakly bound atom carries away a relatively large fraction of the available...

Measurements of kinetic energy release following the unimolecular and collision‐induced dissociation of argon cluster ions, Ar+n, for n in the range 2–60

A double‐focusing mass spectrometer in conjunction with a cluster beam source has been used to measure the average kinetic energy released following the unimolecular and collision‐induced fragmentation (CID) of argon cluster ions. Measurements on unimolecular decay have been made for clusters in the range Ar+5–Ar+60, and for the CID studies the range was Ar+2–Ar+30. Within the observation time window, the kinetic energy release results for the loss of a single argon atom via unimolecular decay are consistent with internal energy being partitioned statistically. Three separate CID routes are identified: (i) loss of one Ar atom; (ii) rapid ( 10−7 s. It is proposed that the CID of small cluster ions proceeds via electronic excitation; but that as the clusters increase in size (n>4) vibrational excitation predominates. A simple spectator model of collisional excitation accounts for the ...

On the origin of metastable decay in Ar+2

Metastable Ar+2 (Ar+2 →Ar++Ar) has been observed in a double‐focusing mass spectrometer from ions created by 70 eV electron bombardment of an Ar cluster beam. New ground and excited state potential energy curves have been calculated for Ar+2, and these have been used to show that metastability is due to radiative decay from the II(1/2)u state of the ion. It is shown that vertical (FC) ionization from neutral Ar2, with a vibrational temperature of approximately 30 K, results in a significant fraction of the ions occupying the II(1/2)u state. Detailed pressure dependent measurements show that collision‐induced dissociation does not contribute to the observed Ar+ signal. The mean kinetic energy released to the Ar+ has been measured as 44 cm−1 in the center‐of‐mass frame, and calculations show that this value is consistent with the proposed mechanism.

The decay dynamics of photoexcited argon cluster ions

Following the photoexcitation of argon cluster ions, Ar+n for n in the range 4–25, kinetic energy release measurements have been undertaken on the fragments using two quite separate techniques. For Ar+4–Ar+6, fragment ion kinetic energy spectra were recorded at 532 nm in a crossed beam apparatus as a function of the angle of polarization of the laser radiation with respect to the incident ion beam. Only Ar+ from Ar+4 was observed to exhibit a polarization dependence together with a comparatively high kinetic energy release. The principal fragment ion Ar+2 was found both to emerge with a low kinetic energy release and to display no dependence on the angle of polarization of the radiation. In a second series of experiments, mass and kinetic energy resolved cluster ions were photodissociated in the entrance to a time‐of‐flight (TOF) device of variable length. The subsequent deflection of all ions allowed for time resolved measurements to be undertaken on the neutral photofragments. Following the absorption o...

Infrared laser‐induced photofragmentation of the positive and negative ions of size‐selected SF6 (NO)n clusters

A detailed study has been undertaken on the infrared photofragmentation of SF6(NO)±n cluster ions. A line‐tuneable CO2 laser has been used to excite the ν3 vibrational mode of the SF6 molecule which is followed by the observation of three separate fragmentation channels: –SF6, –NO, and –2NO. The relative intensities of the fragments are found to be sensitive to the sign of the charge on the ion, the cluster size, and whether n is either odd or even. Within clusters of the same charge, the most marked transitions in fragmentation pattern are found between odd‐ and even‐sized cluster ions, with the decay channels favoring those processes which lead to the formation of even electron ions. There are also large differences in fragmentation pattern between the negatively and positively charged ions. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles and their shapes confirm a pronounced difference in behavior between SF6 (NO)−n and SF6 (NO...

Infrared photofragmentation spectra of size‐selected SF6⋅Ar+n cluster ions

Results are presented of a detailed experimental study of the infrared photofragmentation patterns of size‐selected SF6⋅Ar+n cluster ions for n in the range 3 to 70. Line‐tuneable CO2 and N2O lasers have been used to excited the ν3 vibrational mode of the SF6 molecule which is followed by the loss of one and two argon atoms as the principal fragmentation routes. Which of the two processes is dominant depends quite strongly on the size of the cluster ion concerned, with very pronounced fluctuations in the relative intensities of photofragments being observed for cluster ions in the range SF6⋅Ar+3 to SF6⋅Ar+25. Only for SF6⋅Ar+3 is the fragmentation pattern markedly different from that found for the other ions; an observation that supports an earlier conclusion regarding the relative ionisation energies of the two constituents [Stace et al. J. Phys. Chem. 97, 11363 (1993)]. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles and these h...

Infrared predissociation in cluster ions containing a guest chromophore. A gentle route to the dynamics and spectroscopy of heterogeneous systems

Abstract New observations of the infrared excitation and fragmentation of heterogeneous cluster ions of the form Y·X n + are presented. By ensuring that IP(Y) > IP(X n ) we show that it is possible to study the infrared spectroscopy of neutral chromophores (Y) in mass-selected clusters. By monitoring photofragmentation channels in conjunction with other aspects of cluster ion behaviour, such as intensity fluctuations (the presence of magic numbers), absorption spectra can be related to structure and decay dynamics. Preliminary results are presented for SF 6 7middot;Ar n + , SF 6 ·(CO 2 ) n + , SF 6 ·(NO) n + , and C 2 H 4 ·(NO) n + .

The infrared photofragmentation of Ar+2. Evidence of excited state population from dimer and cluster ionization

Argon dimer ions have been generated via three different techniques: (1) autoionization; (2) vertical ionization of neutral Ar2; (3) ionization and subsequent fragmentation of argon cluster ions. In experiments (2) and (3) the dimers and clusters are formed via the adiabatic expansion of argon in a supersonic beam. In each case Ar+2 ions have been mass selected and subjected to single‐photon infrared excitation (912–1094 cm−1) using a line‐tunable carbon dioxide laser in a crossed‐beam arrangement. Only those Ar+2 ions with internal energies within 1000 cm−1 of a dissociation limit yield Ar+ photofragments, the kinetic energy spread of which has been measured using an electrostatic analyzer. The photofragment kinetic energy spectra of dimer ions formed by autoionization do not exhibit any dependence on the angle of laser polarization; it is proposed that such behavior is due to the presence of a high thermal rotational temperature (500 K). In contrast, the corresponding spectra of Ar+2 formed via vertical...

Collision-induced vibrational excitation of Ar25+

Abstract Using the supersonic expansion technique in conjunction with a high-resolution mass spectrometer, the unimolecular and collision-induced dissociation of Ar 25 + has been studied. From a comparison of the energetics associated with these two fragmentation steps, it is shown that during a collision in which Ar 25 + has a laboratory-frame kinetic energy of 8000 eV, the ion cluster receives approximately 200 cm −1 of internal excitation. It is concluded that this energy appears as vibrational excitation.

The expulsion of excited state Ar+2 from argon cluster ions

Abstract The metastable (radiative) decay of Ar+2 has been studied following the electron impact ionization of neutral Ar2 and larger argon clusters. It is proposed that under certain experimental conditions, a major fraction of observed decay signal originates from Ar+2 ions in the excited II( 1 2 )u electronic state that have been expelled from larger clusters.