J . Phys. Chem. €993,97, 1312-1317
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Deuterium Isotope Effects in the Photofragmentation of Naphthalene Monocations H. W. Jochims, H. Rasekh, E. Riihl, and H. Baumgartel Institut f i r Physikalische Chemie, Freie Universitat Berlin, Takustrasse 3, 1000- Berlin 33, FRG
S. Leach' Laboratoire de Photophysique Mol6culaire du CNRS, Bat. 21 3, Universith Paris-Sud, 91 405 Orsay, France, and DAMAP, Observatoire de Paris- Meudon, 921 95 Meudon, France Received: August 18, 1992; In Final Form: November 23, 1992
Deuterium isotope effects on the photofragmentation thresholds of naphthalene monocations are studied by photoion mass spectrometry using variable-energy monochromatized synchrotron radiation to photoionize CloHs and CloD8. Kinetic isotope shifts (d-h) in the range 30-780 meV were observed for seven fragmentation reactions in the 15-20-eV region. The physicochemical factors leading to kinetic isotope effects are discussed. The observed kinetic isotope shifts are rationalized, mainly in terms of the RRKM/QET model and the specific reaction mechanism pathways. An astrophysical implication of these results is the possibility of deuterium fractionation of polycyclic aromatic hydrocarbons in the interstellar medium.
I. Introduction An energy-selective study of the photofragmentation of the naphthalene monocation was previously carried out on the deuterated compound CloDs using the threshold photoelectronphotoion coincidence (T-PEPICO) technique' and, as excitation source over the range 8-35 eV, monochromatized synchrotron radiation from the LURE-ACO storage ring a t Orsay. Twelve fragmentation reactions were observed within the ion mass range 75-140 amu at E,,, 6 21 eV. The observed product ions are CloD7+, CloD6'; C&+, CsD5+; C7D5+, C7D4+, C7D3+; C6D7+, C&,+, C6D5+,C6D4+,and C6D3+. The appearance potentials of these ions fell into two groups: the three ions CloD7+,CgD6+,and C6D6+have appearance potentials (AP) close to 16 e v , the other ions appearing above 17.7 eV. Fragment ions of lower mass are also produced above 19 eV (see later) but not in large quantities below 21 eV.I Particular attention was paid to the metastable transition reaction CloDs+ CsD6+ C2D2. The CsD6+ appearance potential was observed a t 16.0 0.1 eV. The decay constants for this reaction were measured over the excitation energy range 16-17.7 eV, and the results fitted to RRKM calculated rates. Details of the RRKM calculation, including the choice of vibrational frequencies of the ground and transition states of the molecular ion, are given elsewhere.' Similar RRKM calculations were carried out, as explained in the Appendix to ref 1, for the a d o g o u s reaction of hydrogenated naphthalene: CloHg+ CgH6+ + C2H2, for parent ion internal energy contents 6-10 eV. The results suggested that one could expect the appearance potential of the CgH6+ ion to be about 0.5 eV less than the CsD6' ion value. Electron impact ionization measurements of naphthalene monocation CloH*+ fragmentation appearance potentials have been made by van Brunt and Wacks.2 The observed appearance potential 15.4 0.1 eV for CsH6+is e0.6 eV less than the value measured for CsD6+ in the T-PEPICO experiment. Although this observation is consistent with the predicted isotope effects, it must be remarked that an observed appearance potential is instrument dependent. Its value will in general be higher than the true onset energy (thermochemical dissociation energy) for a specific reaction. The observed appearance potential corresponds to a dissociation rate detectable on the time scale of the particular experimental setup. Since two different techniques were used in the T-PEPICO and electron impact experiments, it is necessary to carry out experiments on the two isotopic species
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using the sameexperimental arrangement in order to demonstrate conclusively that the difference is due, at least in part, to an isotopic dependence of the kinetic shift and not to instrumental factors alone. The present experiments were devised todirectly test deuterium isotope dependence of fragment ion appearance potentials in photoinduced dissociation of the naphthalene monocation. Photoion mass spectrometry was used to measure the relative partial cross sections for production of two groups of ions: CloH7+ (Ciob+), CsH6+ (CsD6+), C6H6+ (C6D6+),and C ~ H S(C7D5+), + whose potentials are in the 15-16-eV range, and the higher AP ions C6H5+ (C6D5+),C5H3+ (C5D3+),and C3H3+ (C,D3+) which appear in the 18-20-eV energy region. Monochromatized synchrotron radiation from the BESSY facility in Berlin was used as the photon excitation source. The results will be shown to throw light on energetic, statistical, and dynamic aspects of ion fragmentation mechanisms. 11. Experimental Section
The photoion mass spectrometry (PIMS) experiments were carried out at BESSY, Berlin, using a experimental setup described fully elsewhere) and summarized here. Synchrotron radiation from the BESSY storage ring is dispersed by a 1-m normal incidence monochromator (McPherson-225). Photon bandwidth was typically 0.2 nm, and photon flux of the order of lo9photons s-). The monochromatized synchrotron radiation is focused into the ion source of a photoion quadrupole mass spectrometer (Leybold Q 200). The naphthalene-hs and -ds are of commercial quality. Introductionof the naphthalenevapor into theion source is via a needle valve. It was important to maintain the same experimental conditions for CloHs and CloDs experiments. The parent and fragment ion counts per unit incident photons were normalized to the 18-20eV photon energy region where the ion signal is maximum. Secondorder radiation was negligible between 15 and 20 eV. 111. Results
The fragment ion intensities for the seven reactions studied were recorded both as linear (I)and logarithmic (log I) values. Figure 1 gives the linear dependence curves for CsH6+ ( m / z = 102) and CsD6+ ( m l z = 108) for naphthalene and deuterated naphthalene, respectively, over the photon excitation energy range E,,, = 15-18 eV. 0 1993 American Chemical Society
Photofragmentation of Naphthalene Monocations
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Energy I eV Figure 2. Photoion mass spectra of naphthalene48 and -d8. Linear plots Of C&I6+and C&,+ ion intensities on the same photon excitation energy scale.
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Energy I eV Figure 1. Photoion mass spectra of naphthalene48 and -ds. Linear plots of product ion intensities: CsH6+ and CpD6'.
The appearance potential (AP) for a particular ion was determined as the point at which the linear I plot reached a background line and was confirmed by the log I plot. This AP therefore includes effects of thermal energy (hot bands, etc) and has only a limited accuracy and physical significance. In particular, it is notoriously difficult to accurately determine the point where the ion intensity reaches the background. More important than accurate APs in the present investigationwas the determination of the difference between appearance potentials for isotopically related fragments. This was carried out by direct comparison of the intensity curves of the two isotopic fragments and involves matching of important sectionsof the intensity plot, rather than a single point as in the AP determinations. Two methods of comparison were used, as follows. A faithful transparent reproduction of the intensity curve for a particular fragment was placed onto the curve for its isotopic species, vertically adjusting the background signal level (beforethreshold) to be equal. Alignment of the energy scale was done in two ways, as illustrated in computerized versions of the two techniques in Figures 2 and 3: (1) With exact alignment of corresponding energies (Figure 2), the difference in the appearance potentials for the two isotopic fragments was determined by measuring the horizontal separation of the isotopic curves at several positions within about a 0.5-eV energy interval near threshold. (2) In the second method the intensity curves were aligned so that the two isotopic curves are confounded in the threshold region (and generally so for 0.5-1 eV above the threshold) (Figure 3). The isotopic kinetic energy shift could thus be read directly from the relativedisplacementof the twoenergy scales. Thesetwomethods are based on the reasonable assumption that the initial portions of the curves, within =0.5 eV of the fragment ion threshold, are similar in growth with photon energy for the two isotopicspecies. The measured appearance potentials and the isotopic energy shifts are effective values and have to be corrected for thermal energy E,h of the isotopic species. The average thermal energy at 300 K is calculated to be 112.2 meV for CloHs and 142.6 meV for C&8. Although one should be concerned with the actual thermal energy distribution, it has been shown that a 6 function
Photon Energy I eV Figure 3. Photoion mass spectra of naphthalene-h8 and -d8. Aligned linear plots of C8H6+ and CsD6+ ion intensities (see text). Upper and lower horizontal scales refer to C&+ and CgH6+, respectively.
is sufficient at high internal energies: so that thermal energy effects can reasonably be represented by the average thermal energy. The total excitation energy at the fragment appearance potential is thus approximately equal to the observed appearance potential plus the average thermal energy. The kinetic isotope shifts (d-h) were corrected for the isotopicdifference in average thermal energies by adding 30 meV to the measured values. The measured appearance potentials(considered to be accurate to within 0.05-0.1 eV), and corrected kinetic isotope shifts are given in Table I. The energy shift values given by the alignment methods are averagesof the two methods discussed above. These values are considered to be more accurate (
