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Laser photodissociation of fluorinated molecular dications
Chemical Physics ELSEVIER
Chemical Physics 190 (1995) 123-130
Laser photodissociation of fluorinated molecular dications Stephen D. Price a, Yin-Yu Lee b.c, Michelle Manning bxa, Stephen R. Leone b,c,2 " Chemistry Department, University College London, 20 Gordon Street, London WCIH OAJ, UK Joint Institute for Laboratory Astrophysics, National Institute of Standards and Technology and University of Colorado, Boulder, CO 80309, USA c Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Received 22 July 1994
Abstract The results of experiments are reported to identify the ionic photoproducts produced by the interaction of gaseous dications, CF 2+, SF 42+, SF 32+, SF 22+, and SF 24, with tunable pulsed laser radiation in the visible and at the fixed wavelengths of 355, 532, and 1064 nm. The experiments show that CF~ ÷ , SF2+ , SF22+ and SF2÷ readily absorb visible light and are promoted to excited states which dissociate by both neutral-loss and charge separation. Detailed studies of CF32÷ show that the dissociation occurs by single-photon absorption which promotes the molecule to unbound excited states. Excitation thresholds for the photoinduced neutral-loss and charge separation reactions of SF2+ and SF2÷ are apparent from the data. A qualitative interpretation of the competition between the neutral loss and charge separation processes is discussed in terms of a simple model of the potential energy surfaces.
I. Introduction A detailed picture of the structure and properties of molecular singly charged ions is provided by experiments that study the interaction of these species with electromagnetic radiation. However, similar investigations of the optical spectroscopy and photochemistry of molecular dipositive ions are more difficult to perform. These difficulties are due principally to the shortlived and reactive nature of most electronic states of molecular dications [ 1,2]. Reports of the optical spectroscopy of molecular dications have appeared sporadically in the literature. Despite attempts to study several systems, resolved Present address: Department of Chemistry, University of Wisconsin-Madison, Madison, WI 56706, USA. 2 Staff member, Quantum Physics Division, National Institute of Standards and Technology. 0301-0104/95 / $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0301-0104(94)00318-1
emission spectra have only been assigned and interpreted for N 2÷ and NO 2÷ [ 3 - 6 ] . Reports of photoabsorption by molecular dications are similarly restricted to N 2+ and NO 2÷ . Several studies are available for the bound ~ quasi-bound absorption spectrum of N 2÷ using ion photofragment spectroscopy; these investigations provide a detailed picture of the structure and dissociation dynamics of this dication [ 7 - 1 2 ] . The technique of ion photofragment spectroscopy involves the detection of singly charged fragment ions, formed by the predissociation of an excited electronic state, as a sensitive monitor of photoabsorption to that excited state, N22 + (X
1~; ) _~photon ~
-+N+ +N + .
N22 + ( A 1Hu) (1)
For N 2 + these bound ~ quasi-bound transitions are superimposed on a continuous absorption spectrum
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S.D. Price et al. / Chemical Physics 190 (1995) 123-130
attributed to transitions involving excitation to unbound excited states [2]. The only report of photoabsorption by a molecular dication other than N22+ involves the detection of the singly charged fragment ions formed following laser excitation of NO 2+ from its ground state to a continuum level above the potential energy well of the A (21-I) state [ 13 ] : N O 2 + ( X 2~ + ) + photon --* N O 2 + ( A 2II )
~N++O + .
(2)
Thus, the only photochemical behavior of molecular dications observed to date involves the promotion of N 2+ and NO 2+ to excited electronic states which decay via charge separation (Eqs. ( 1) and (2)). This paper reports the results of experiments to investigate the interaction of several polyatomic dications, CF23+ , SF22+ , SF2 + and SF2 +, and the diatomic d i c a t i o n S F 2 +, with laser radiation. The primary aim of these experiments is to perform the first assessment of polyatomic dication photochemistry. The results presented below show that four of these dications absorb strongly in the visible region of the electromagnetic spectrum, an observation which suggests that dication photoabsorption is not as rare as the available
data for diatomic species might imply [ 2]. The absorption processes observed involve single photons, and the excited states that are populated by photoexcitation eventually decay by neutral-loss reactions (Eq. (3)) in addition to charge separation (Eq. (4)). XF2+ + photon ~
2+ "~nF,
(3)
XF m_ n
XF2m+ + photon ~ XF~_ 1 + F ÷ •
(4)
These results are, to our knowledge, the first report of photoabsorption by a polyatomic dication and the first reported observations of photoinduced neutral-loss reactions of dications. The occurrence of both neutralloss and charge separation following the photoexcitation of these fluorinated dications is strikingly similar to the decay of these ions induced by collisional excitation [ 14].
2. Experimental To investigate the photofragmentation of molecular dications we adapted the experimental apparatus which was previously used to generate dication beams for collisional studies [14-17]. As illustrated in Fig. 1, ions are formed by electron impact ionization of an Electron multiplier ion detectol
TIME" OF- FLIGH" MASS SPECTROME Einz¢l lens Deflector Dlcatior beam 3.. . . . . . q
Electrc ~ n mp°ct ion jsourc¢ Diffusion pump
Laser / interaction region
[+ ) :ion
:~'_"_ _ , _~_~ ~ Repeller plate
Faraday cup
Diffusion pump
Fig. l. Schematic diagram of the apparatus used to investigate the photodissociation reactions of molecular dieations.
S.D. Price et al. / Chemical Physics 190 (1995) 123-130
appropriate precursor molecule (CF4 o r S F 6 ) and a dication beam is generated from these ions by mass selection using a quadrupole mass spectrometer (QMS). The ion beam emerging from the QMS is focused into the source region of a time-of-flight mass spectrometer (TOFMS), where it is intersected at right angles by a pulsed ( 10 Hz) beam of tunable or fixed frequency laser radiation (Fig. 1 ). The laser radiation is generated using a N d / Y A G pumped dye laser and, for experiments at higher photon fluxes but fixed frequencies, the harmonics of the Nd/YAG laser are also used. The direct YAG beam has a 'doughnut' intensity profile and is collimated to give a spot diameter of = 1.5 cm. This comparatively large spot size limits the energy density to less than 0.5 mJ mm - 2 pulse- ~, which eliminates the contribution of multiphoton phenomena to the photofragment yields. When the ion beam is not being sampled, the source region of the TOFMS is maintained in a field-free state at a potential of 0 V. In this arrangement, the electrical potential of the ion source defines the transverse velocity of the dications where they cross the laser beam. A low ion beam velocity maximizes the density of the ions that interact with the laser radiation and hence maximizes the photodissociation signal. The experiments described in this report were carried out at a dication beam energy of 49 eV; this is the lowest energy at which the experiment generates usable ion fluxes. Following the interaction of the pulse of laser radiation with the ion beam, the repeller plate of the TOFMS is pulsed to a positive voltage to accelerate any photofragments as well as the undissociated dications into the TOFMS for detection and identification. In our experimental arrangement the ions in the beam possess a significant velocity component ( = 1170 m s - ~for CF~ '- ) perpendicular to the axis of the TOFMS. Hence, the region of interaction of the laser radiation with the ion beam must be situated about 3 cm before the center of the source region of the TOFMS to allow the ions generated in the photodissociation events to follow the parabolic trajectories illustrated in Fig. 1 and pass through the acceleration region and into the drift region of the TOFMS. Deflection electrodes in the drift region of the TOFMS (Fig. 1 ) are used to nullify the perpendicular component of velocity due to the ion beam, allowing the ions to reach the detector of the TOFMS. As has been discussed before, these deflection electrodes introduce some discrimination into the mass
125
spectra recorded on the TOFMS [14-17]. For the experiments described in this publication, the deflectors were set at a voltage optimized for the transmission of the dicationic products of the photodissociation processes. In the absence of mass discrimination effects from other sources, the relative ion yields of the other photofragments observed in the mass spectra can be generated from the recorded intensities by scaling with experimentally generated correction factors [ 15]. In the period between the laser pulses the dication beam passes through the source region of the TOFMS and is incident upon a Faraday cup to determine the beam current for normalization purposes.
3. Results Fig. 2a shows a product ion mass spectrum recorded following interaction of a beam of CF] + ions with laser radiation at 532 nm. A CF22÷ signal is clearly observed in the mass spectrum at an m/z = 25. Fig. 2b shows a mass spectrum recorded with the laser radiation blocked, but under otherwise identical conditions to Fig. 2a. It is immediately apparent that no CF22+ signal is present in this background spectrum. Hence, the formation of CF22÷ from CF~ + can be definitively
20000
(o)
c~ ÷ x5
I
x5
I0000 F+
._w '~
C
L
k
(b)
CF~÷ 2000C
I0000
°,o
x5
~b
t
I
3b
4~o
x5
'sb " '6b
Moss number
Fig. 2. (a) Mass spectrum recorded following the interaction of a beam of CF32÷ with laser radiation at 532 nm. (b) Mass spectrum recorded under identical conditions to the spectrum in (a) but with the laser radiation blocked.
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S.D. Price et al. / Chemical Physics 190 (1995) 123-130
assigned to a photon induced neutral-loss process, CF32+ + photon ~ CF 2÷ + F .
(5)
Fig. 2a also shows the presence o f f ÷ and CF~- ions following the interaction of the CF 2÷ ions with the laser radiation. Clearly these singly charged ions can be assigned to the products of photoinduced charge separation, CF~ ÷ + photon ~ CF~- + F ÷ .
(6)
Note also that very weak F ÷ and CF~- signals are observed in the background mass spectrum (Fig. 2b). These ions result from the continuous unimolecular dissociation of the metastable CF~ ÷ parent ions in the beam. It is interesting to observe that no unimolecular neutral-loss processes are observed, since no CF 2+ ions are detected in the background mass spectrum (Fig. 2b). It is also important to note the differences in peak shapes observed in the time of flight mass spectrum (Fig. 2a). The peak corresponding to the CF 2÷ ions is narrow, which indicates there is only a small energy release involved in the formation of this ion from CF~ ÷ . However, the F ÷ peak is wide, double peaked and asymmetric, and the CF~- peak is also wide. The width of these peaks indicates that the F ÷ and CF~ions are formed from CF32 ÷ with a considerable kinetic energy release. The energy release difference between the neutral-loss and charge separation channels is not surprising, since the forces involved in the neutral-loss and charge separation processes are markedly different. The observed width of the F ÷ peak corresponds to the formation of at least some of these ions from CF32÷ with an energy greater than 4 eV. The double-peaked shape of the F ÷ ion signal could arise from either the incomplete collection of these energetic ions or an anisotropic F ÷ velocity distribution generated by photoabsorption of the polarized laser radiation. However, we observe no significant change in the F ÷ peak shape as the orientation of the laser polarization vector is varied with respect to the ion beam direction. Hence, we consider the doubled and asymmetric F ÷ peak shape to result from the incomplete collection of the energetic ions by the TOFMS, a conclusion confirmed by simulations presented below. To explain the F ÷ peak shapes in detail requires consideration of the velocity components of the ions both parallel and perpendicular to the axis of the
TOFMS and the geometry of the TOFMS itself. The significant kinetic energy imparted to the F ÷ ions in the photoinduced charge separation can result in some F ÷ ions not reaching the detector, particularly when the resulting initial velocity vector has a significant component perpendicular to the axis of the TOFMS. Ions with significant velocity components perpendicular to the TOFMS axis would be detected at flight times close to the center of the flight time distribution for an ion of m/z = 19. Hence, the selective loss of these energetic ions results in the doubled peak shape. Additionally, the asymmetry of the F + peak is due to the optimization of the apparatus for collection of the doubly charged photoproducts; in this configuration an aperture (12 mm) at the entrance to the drift region stops some of the energetic F ÷ ions from reaching the detector (Fig. 1). The losses due to this restriction are more severe for ions formed with a significant impulse directed away (downward) from the TOFMS axis resulting in the asymmetric peak shape. The above qualitative arguments are borne out by detailed Monte Carlo simulations of the expected F ÷ peak shape. As shown by the solid line in Fig. 3, these simulations satisfactorily reproduce the experimental peak shape (bars) when the kinetic energy distribution of the F ÷ ions is modeled by a Gaussian distribution centered at an energy release of 4.5 eV with a full width
I
Illl
[IIz ~
8.99
9.98 Time of
flight /;~s
Fig. 3. Comparison of the experimental (error bars) and simulated (solid line) F + peak shape in the time of flight mass spectrum recorded following photodissociation of CF 2+ at 532 nm. The tail apparent on the peak to longer time of flight is attributed to the rise time of the ion extraction pulse, a factor not included in the simulation. See text for details.
S.D. Price et al. / Chemical Physics 190 (1995) 123-130
127
Table 1 Photofragments detected following the interaction of perfluorinated molecular dications with laser radiation Parent ion
CF] + SF 2÷ SF~ + SF 2 +
SF~ +
Photofragments detected
CF~ + F +, CF~ S 2+ F +, S ÷ SF 2+ F +, SF + SF 2 ÷ SF~ ÷ F +, SF~none
Photodissociation process
CF32+ ~ C F ~ + + F CF32÷ ~ C F ~ - + F ÷ SP e+ --->S 2÷ + F SF 2+ ~ S + + F + SF~ + ~ SF 2+ + F SF~ + ~ SF ÷ + F ÷ SF32÷ --->SF 2 + + 2F SF~ + ~ S F ~ ÷ + F SF~ + ~ S F ~ - + F ÷ -
Relative ion yield a 355 nm
532 nm
1064 nm
4.3 3.0 0.96 2.3 0.90 0.88 0.30 5.6 4.1 -
2.4 1.6 0.093 1.3 0.37 0.36 < 0.003 1.7 0.62
0.25 0.2 < 0.003 <0.012 0.027 0.040 < 0.003 0.33 <0.012
The photodissociation reactions producing the given photofragments are listed, as well as the relative ion yields of each photodissociation process. a As described in the text, the relative ion yields have been corrected for mass discrimination effects and the variation in the photon density as the wavelength of the laser is varied. The error limits for the ion yields are + 9 % for the neutral-loss processes and + 2 5 % for the charge separation reactions. The corrections for the singly charged ion signals for the losses in the apparatus introduce a significantly larger uncertainty in those values.
at half maximum (FWHM) of 2.5 eV. The 'doughnut' laser intensity distribution also contributes to the peak shape, and is modeled by restricting photodissociation events to a tubular shaped volume of inner radius 3 mm and outer radius 8 mm, oriented parallel to and centered on the laser propagation axis. The beam of parent dications is assumed to have a Gaussian intensity profile (FWHM of 5 mm) perpendicular to its direction of propagation. It is satisfying to note that under identical conditions the simulation also satisfactorily reproduces the CF~- peak shape using an ion energy distribution derived from the fitted F ÷ kinetic energy distribution by conservation of energy and momentum. These computer simulations also indicate that the F + peak shape should be sensitive to the position of the laser interaction region and the deflector setting, as we observe experimentally. The simulations show that with the experimental setup described above we detect = 20% of the F ÷ ions and = 3 5 % of the CF~- ions. Under identical conditions, formation of CF 2÷ with an energy release of = 100 meV results in collection of all the dications, with a narrow mass spectral peak, again as observed experimentally (Fig. 2a). Further experiments confirm that the photodissociation behavior observed for CF~ ÷ is common to other perfluorinated dications. In these experiments, we monitored the ionic products formed following the inter-
action of SF 2+, SF~ ÷ , SF~ ÷ , and SF~ ÷ with laser radiation at 355, 532, and 1064 nm. All the observed dication photodissociation processes, at three laser wavelengths, are listed in Table 1. For SF ÷ , SF~ + , and SF 2 + a behavior similar to that of CF~ ÷ is observed, with both neutral-loss and charge separation processes being induced by the laser photons. However, one notable difference is that SF~ ÷ can lose two fluorine atoms to form SF 2÷. No photoinduced reactions were observed for SF~+; presumably the first allowed absorption for this dication is at wavelengths shorter than 355 nm, the largest available photon energy. Table 1 also shows estimates of the relative ion yields for the different photodissociation channels for each dication. These values have been obtained from the mass spectral intensities with appropriate correction for all the discrimination effects of the apparatus as described above and the different photon densities at each laser wavelength. Hence, the relative magnitudes of these relative ion yields provide an estimate of the relative magnitudes of the photodissociation probability for different photodissociation channels. Due to the low dication currents generated for the sulfur species, and the fact that sulfurous deposits from the SF6 precursor gas rapidly contaminated the ion source, further investigations of the photodissociation phenomena were restricted to CF~ +. We studied the
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S.D. Price et al. / Chemical Physics 190 (1995) 123-130
fluctuations in the laser power and ion beam intensity, over a laser wavelength range from 556 to 790 nm and with a linewidth of 1 cm- 1.
50 45 40" •~, 35
~
0
6
4
n
m
4. Discussion
~ " 3.0_J 25
20-
15 05
~
,.~
F* yield at 562nm
i .... 10
i .... 1.5
i .... 2.0
i 2.5
Log (Laser power) Fig. 4. Dependence of the yield of the photoinduced neutral-loss and charge separation processes ofCF~ + on laser power at 562 and 1064 nm. The slopes are 0.99 + 0.07 and 1.02-1-0.06 for CF~ + yield at 562 and 1064 nm, respectively. The slope f o r F ÷ yield at 562 nm is 0.98 + 0.15. The neutral-loss process was monitored by detecting CF~ + ions, while the charge separation channel was monitored by detecting F ÷ ions.
dependence on the laser power of the two photoinduced reactions (Eqs. (5) and (6)) for CF 2÷ . These experiments were performed by monitoring the photoproduct intensities while attenuating the laser beam with a wire mesh. As shown in Fig. 4, the power dependence of the ion yield confirms that multiphoton processes are not involved in the formation of either the CF 2+ or F ÷ photoproducts. We also recorded a dication photofragmentation spectrum by monitoring the photodissociation signal for both the neutral-loss and charge separation reactions ofCF~ + (Eqs. (5) and (6)) as the laser wavelength is varied. However, no spectral structure was observed in either the CF~ ÷ or F + signal, when corrected for
Recent work has shown that perfluorinated molecular dications exhibit a new form of collision-induced reactivity involving neutral-loss reactions [ 14], for example, CF 2+ + M - - * C F f + F + + M .
(7)
It is clear from the results presented above that photoinduced neutral-loss processes (Eq. (5)) also play an important role in the photochemistry of perfluorinated molecular dications. In the case of CF 2 +, since we observe no spectral structure (at the experimental resolution of = 1 c m - 1) in the photoinduced ion signal, it seems likely that both the neutral-loss and charge separation reactions result from bound--* free transitions. Indeed, similar transitions [2] have been observed for N 2+ and N O 2 + . In fact, both the photoinduced processes observed for CF~ + (Eqs. (5) and (6)) may arise from population of the same initial electronic state since, within the experimental error, the ratio of the ion yields of the two photochannels for CF 2+ (Table 2) does not vary with wavelength. This concept, that both the photoinduced neutral-loss and charge separation processes arise from the same excited electronic state, is consistent with a previously proposed model for the low-lying potential surfaces of perfluorinated molecular dications and CH3I 2+ [ 14,18 ]. This model was developed to explain
Table 2 Ratios of the relative ion yields for photoinduced neutral-loss and charge separation at three different laser wavelengths Parent ion
Neutral-loss to charge separation ratio 355 nm
CF3z+ SF 2 + SF~ + SF~ + "
532 nm
1064 nm
max
min
max
min
max
min
2.1 0.70 1.5 2.0
1.0 0.34 0.73 1.0
2.2 0.10 1.51 4.0
1.1 0.05 0.74 2.0
1.9 1.0 -
0.9 0.48 -
The m a x i m u m and m i n i m u m values of the ratios consistent with the experimental uncertainties in the relative ion yield (Table 1 ) are given. "The ion yield for formation of SF 2+ has been used to calculate these ratios.
S.D. Price et al. / Chemical Physics 190 (1995) 123-130
CF#*÷ F l E uJ
....
Et
CF2*+ F+ 1 r (CF2--FI Fig. 5. Schematic potential energy surfaces for a proposed electron transfer mechanism which allows competition of neutral-loss and charge separation from the same electronic state of CF~ ÷ following excitation. See text for details.
the competition between neutral-loss and charge separation reactions observed following collisional excitation of these species. As illustrated in Fig. 5, this model assumes that the initial excitation accesses a repulsive charge separating state, but as the ions separate there may be a curve crossing with an electronic state correlating with the neutral-loss asymptote. As has been discussed previously, the energetics of perfluorinated molecular dications may result in such a curve crossing at an interspecies separation where the coupling between these electronic states causes the neutral-loss and charge separating reactions to compete effectively [ 14]. Indeed, if this model is correct, it is tempting to conclude that the same electronic state of CF 2+ is populated in the photoexcitation and collisional excitation experiments. The curve crossing model explains why, in the absence of photoexcitation or collisional excitation, the only unimolecular decay pathway of the long-lived dications (e.g. CF 2+ ) in the ion beam involves charge separation (Fig. 2b). This is because, as is apparent from Fig. 5, the long-lived dication states will lie at energies below the curve crossing and, in the absence of additional excitation, they are unable to reach the asymptote corresponding to neutralloss. From the curve crossing model we can also predict that there exists an excitation threshold (Et in Fig. 5) below which the charge separation reaction should still occur but neutral-loss reactions would not be observed. As discussed below, this effect is observed for some of the SF] ÷ photodissociation reactions listed in Table 1. The study of the interaction of SF] + with laser radiation provides further indication that the same electronic states may determine the behavior of
129
perfluorinated dications in both collisional excitation and photoexcitation experiments. Collisional studies show that for SF42+ there is an activation energy in excess of 3 eV for the neutral-loss process [ 14]. This activation energy was tentatively interpreted as the excitation energy required to collisionally promote the ion to the electronic state responsible for dissociation (Fig. 5). Hence, if the same electronic states are responsible for both the photoinduced and collision induced reactivity, we would not expect SF42+ to exhibit any photoinduced reactivity at the photon energies employed in this study. This is indeed observed experimentally. The simple curve crossing model also is consistent with the observation of the photodissociation pathways f o r S F 2 + and SF 2+. For SFz2+, as for CF z +, the ratios of the ion yields of the neutral-loss and charge separation photodissociation reactions (Table 2) remain constant as the wavelength is varied. For SF z+ we observe (Table 1 ) that the threshold for both photodissociation channels lies between 532 and 1064 nm, and as the threshold is approached, the relative importance of the charge separation process increases (Table 2). As described above, such a trend is predicted by considering the energetics of the simple curve crossing model (Fig. 5). However, more subtle effects, such as the effect of the changing velocity through the crossing at lower excitation energies, could also enhance the nonadiabatic pathway. For SF 2 +, the data indicate that the threshold for the double neutral-loss reaction to form SF z + lies between 355 and 532 nm; whereas, single neutral-loss to form SF 2+ and charge separation are both observed at 532 nm. This indicates that at 355 nm at least two electronic states of SF~ + are accessed, one of which dissociates by double neutral loss to form SF 2 + and is inaccessible at 532 nm. The threshold for the charge separation reaction of SF~ + appears to lie between 532 and 1064 nm, whereas the threshold for single neutral-loss to form SF22+ appears to lie above 1064 nm. This threshold behavior is the opposite of expectation if both the charge separation reaction and the neutral-loss processes result from the decay of the same electronic state via the curve crossing mechanism. One explanation for this behavior is that the energetics of SF 2+ place the relevant curve crossings (Fig. 5) at interspecies separations where the coupling between the electronic states does not permit a competition between neutral-loss and
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S.D. Price et al. / Chemical Physics 190 (1995) 123-130
charge separation. In any case, it appears that for SF~ + there m a y be at least three different electronic states which can be populated by photoexcitation, and each electronic state dissociates to different products.
5. Conclusion W e observe several n e w photoinduced reactions of fluorinated molecular dications i n v o l v i n g neutral-loss to produce a daughter dication. These processes compete with photoinduced charge separation following absorption of a single photon of visible radiation. The results provide the first observation of photoabsorption by a polyatomic dication and the first observation of a photoinduced neutral-loss reaction of a dication.
Acknowledgement The authors gratefully acknowledge the Air Force Office of Scientific Research (High E n e r g y Density Matter Program) for their generous support of this research.
References [ 1] D. Mathur, Phys. Rept. 225 (1993) 193.
[2] M. Larrson, Comments At. Mol. Phys. 29 (1993) 39. [3] P.K. Carrol, Can. J. Phys. 36 (1958) 1585. [4] D. Cossart, F. Launay, J.M. Robbe and G. Gandara, J. Mol. Spectry. 113 (1985) 142. [5] D. Cossart, C. Cossart-Magos and F. Launay, J. Chem. Soc. Faraday Trans. 87 ( 1991 ) 2525. [6] M.J. Besnard, L. Hellner, Y. Malinovich and G. Dujardin, J. Chem. Phys. 85 (1986) 1316. [7] P.C. Cosby, R. Moiler and H. Helm, Phys. Rev. A 28 (1983) 766. [8] T.E. Masters and P.J. Sarre, J. Chem. Soc. Faraday Trans. 86 (1990) 2005. [9] D.M. Szaflarski, A.S. Mullin, K. Yokoyama, M.N.F. Ashfold and W.C. Lineberger, J. Chem. Phys. 95 ( 1991 ) 2122. [ 10] A.S. Mullin, D.M. Szaflarski, K. Yokoyama, G. Gerber and W.C. Lineberger, J. Chem. Phys. 96 (1992) 3636. [ 11 ] M. Larsson, G. Sundstrom, L. Brostrom and S. Mannervik, J. Chem. Phys. 97 (1992) 1750. [ 12] G. Sundstrom, M. Carlson, M. Larsson and L. Brostrom, Chem. Phys. Letters 218 (1994) 17. [ 13] L.G.M. Petterson, L. Karlsson, M.P. Keane, A. Naves de Brito, N. Correia, M. Larrson, L. Brostrom, S. Mannervik and S. Svensson, J. Chem. Phys. 96 (1992) 4884. [ 14] S.D. Price, M. Manning and S.R. Leone, Chem. Phys. Letters 214 (1993) 553. [15] S.A. Rogers, S.D. Price and S.R. Leone, J. Chem. Phys. 98 (1993) 280. [16] S.D. Price, S.A. Rogers and S.R. Leone, J. Chem. Phys. 98 (1993) 9455. [17] M. Manning, S.D. Price and S.R. Leone, J. Chem. Phys. 99 (1993) 8695. [ 18] J.M. Curtis, A.G. Brenton, J.H. Beynon and R.K. Boyd, Chem. Phys. 117 (1987) 325.
Chemical Physics 190 (1995) 123-130
Laser photodissociation of fluorinated molecular dications Stephen D. Price a, Yin-Yu Lee b.c, Michelle Manning bxa, Stephen R. Leone b,c,2 " Chemistry Department, University College London, 20 Gordon Street, London WCIH OAJ, UK Joint Institute for Laboratory Astrophysics, National Institute of Standards and Technology and University of Colorado, Boulder, CO 80309, USA c Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Received 22 July 1994
Abstract The results of experiments are reported to identify the ionic photoproducts produced by the interaction of gaseous dications, CF 2+, SF 42+, SF 32+, SF 22+, and SF 24, with tunable pulsed laser radiation in the visible and at the fixed wavelengths of 355, 532, and 1064 nm. The experiments show that CF~ ÷ , SF2+ , SF22+ and SF2÷ readily absorb visible light and are promoted to excited states which dissociate by both neutral-loss and charge separation. Detailed studies of CF32÷ show that the dissociation occurs by single-photon absorption which promotes the molecule to unbound excited states. Excitation thresholds for the photoinduced neutral-loss and charge separation reactions of SF2+ and SF2÷ are apparent from the data. A qualitative interpretation of the competition between the neutral loss and charge separation processes is discussed in terms of a simple model of the potential energy surfaces.
I. Introduction A detailed picture of the structure and properties of molecular singly charged ions is provided by experiments that study the interaction of these species with electromagnetic radiation. However, similar investigations of the optical spectroscopy and photochemistry of molecular dipositive ions are more difficult to perform. These difficulties are due principally to the shortlived and reactive nature of most electronic states of molecular dications [ 1,2]. Reports of the optical spectroscopy of molecular dications have appeared sporadically in the literature. Despite attempts to study several systems, resolved Present address: Department of Chemistry, University of Wisconsin-Madison, Madison, WI 56706, USA. 2 Staff member, Quantum Physics Division, National Institute of Standards and Technology. 0301-0104/95 / $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0301-0104(94)00318-1
emission spectra have only been assigned and interpreted for N 2÷ and NO 2÷ [ 3 - 6 ] . Reports of photoabsorption by molecular dications are similarly restricted to N 2+ and NO 2÷ . Several studies are available for the bound ~ quasi-bound absorption spectrum of N 2÷ using ion photofragment spectroscopy; these investigations provide a detailed picture of the structure and dissociation dynamics of this dication [ 7 - 1 2 ] . The technique of ion photofragment spectroscopy involves the detection of singly charged fragment ions, formed by the predissociation of an excited electronic state, as a sensitive monitor of photoabsorption to that excited state, N22 + (X
1~; ) _~photon ~
-+N+ +N + .
N22 + ( A 1Hu) (1)
For N 2 + these bound ~ quasi-bound transitions are superimposed on a continuous absorption spectrum
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S.D. Price et al. / Chemical Physics 190 (1995) 123-130
attributed to transitions involving excitation to unbound excited states [2]. The only report of photoabsorption by a molecular dication other than N22+ involves the detection of the singly charged fragment ions formed following laser excitation of NO 2+ from its ground state to a continuum level above the potential energy well of the A (21-I) state [ 13 ] : N O 2 + ( X 2~ + ) + photon --* N O 2 + ( A 2II )
~N++O + .
(2)
Thus, the only photochemical behavior of molecular dications observed to date involves the promotion of N 2+ and NO 2+ to excited electronic states which decay via charge separation (Eqs. ( 1) and (2)). This paper reports the results of experiments to investigate the interaction of several polyatomic dications, CF23+ , SF22+ , SF2 + and SF2 +, and the diatomic d i c a t i o n S F 2 +, with laser radiation. The primary aim of these experiments is to perform the first assessment of polyatomic dication photochemistry. The results presented below show that four of these dications absorb strongly in the visible region of the electromagnetic spectrum, an observation which suggests that dication photoabsorption is not as rare as the available
data for diatomic species might imply [ 2]. The absorption processes observed involve single photons, and the excited states that are populated by photoexcitation eventually decay by neutral-loss reactions (Eq. (3)) in addition to charge separation (Eq. (4)). XF2+ + photon ~
2+ "~nF,
(3)
XF m_ n
XF2m+ + photon ~ XF~_ 1 + F ÷ •
(4)
These results are, to our knowledge, the first report of photoabsorption by a polyatomic dication and the first reported observations of photoinduced neutral-loss reactions of dications. The occurrence of both neutralloss and charge separation following the photoexcitation of these fluorinated dications is strikingly similar to the decay of these ions induced by collisional excitation [ 14].
2. Experimental To investigate the photofragmentation of molecular dications we adapted the experimental apparatus which was previously used to generate dication beams for collisional studies [14-17]. As illustrated in Fig. 1, ions are formed by electron impact ionization of an Electron multiplier ion detectol
TIME" OF- FLIGH" MASS SPECTROME Einz¢l lens Deflector Dlcatior beam 3.. . . . . . q
Electrc ~ n mp°ct ion jsourc¢ Diffusion pump
Laser / interaction region
[+ ) :ion
:~'_"_ _ , _~_~ ~ Repeller plate
Faraday cup
Diffusion pump
Fig. l. Schematic diagram of the apparatus used to investigate the photodissociation reactions of molecular dieations.
S.D. Price et al. / Chemical Physics 190 (1995) 123-130
appropriate precursor molecule (CF4 o r S F 6 ) and a dication beam is generated from these ions by mass selection using a quadrupole mass spectrometer (QMS). The ion beam emerging from the QMS is focused into the source region of a time-of-flight mass spectrometer (TOFMS), where it is intersected at right angles by a pulsed ( 10 Hz) beam of tunable or fixed frequency laser radiation (Fig. 1 ). The laser radiation is generated using a N d / Y A G pumped dye laser and, for experiments at higher photon fluxes but fixed frequencies, the harmonics of the Nd/YAG laser are also used. The direct YAG beam has a 'doughnut' intensity profile and is collimated to give a spot diameter of = 1.5 cm. This comparatively large spot size limits the energy density to less than 0.5 mJ mm - 2 pulse- ~, which eliminates the contribution of multiphoton phenomena to the photofragment yields. When the ion beam is not being sampled, the source region of the TOFMS is maintained in a field-free state at a potential of 0 V. In this arrangement, the electrical potential of the ion source defines the transverse velocity of the dications where they cross the laser beam. A low ion beam velocity maximizes the density of the ions that interact with the laser radiation and hence maximizes the photodissociation signal. The experiments described in this report were carried out at a dication beam energy of 49 eV; this is the lowest energy at which the experiment generates usable ion fluxes. Following the interaction of the pulse of laser radiation with the ion beam, the repeller plate of the TOFMS is pulsed to a positive voltage to accelerate any photofragments as well as the undissociated dications into the TOFMS for detection and identification. In our experimental arrangement the ions in the beam possess a significant velocity component ( = 1170 m s - ~for CF~ '- ) perpendicular to the axis of the TOFMS. Hence, the region of interaction of the laser radiation with the ion beam must be situated about 3 cm before the center of the source region of the TOFMS to allow the ions generated in the photodissociation events to follow the parabolic trajectories illustrated in Fig. 1 and pass through the acceleration region and into the drift region of the TOFMS. Deflection electrodes in the drift region of the TOFMS (Fig. 1 ) are used to nullify the perpendicular component of velocity due to the ion beam, allowing the ions to reach the detector of the TOFMS. As has been discussed before, these deflection electrodes introduce some discrimination into the mass
125
spectra recorded on the TOFMS [14-17]. For the experiments described in this publication, the deflectors were set at a voltage optimized for the transmission of the dicationic products of the photodissociation processes. In the absence of mass discrimination effects from other sources, the relative ion yields of the other photofragments observed in the mass spectra can be generated from the recorded intensities by scaling with experimentally generated correction factors [ 15]. In the period between the laser pulses the dication beam passes through the source region of the TOFMS and is incident upon a Faraday cup to determine the beam current for normalization purposes.
3. Results Fig. 2a shows a product ion mass spectrum recorded following interaction of a beam of CF] + ions with laser radiation at 532 nm. A CF22÷ signal is clearly observed in the mass spectrum at an m/z = 25. Fig. 2b shows a mass spectrum recorded with the laser radiation blocked, but under otherwise identical conditions to Fig. 2a. It is immediately apparent that no CF22+ signal is present in this background spectrum. Hence, the formation of CF22÷ from CF~ + can be definitively
20000
(o)
c~ ÷ x5
I
x5
I0000 F+
._w '~
C
L
k
(b)
CF~÷ 2000C
I0000
°,o
x5
~b
t
I
3b
4~o
x5
'sb " '6b
Moss number
Fig. 2. (a) Mass spectrum recorded following the interaction of a beam of CF32÷ with laser radiation at 532 nm. (b) Mass spectrum recorded under identical conditions to the spectrum in (a) but with the laser radiation blocked.
126
S.D. Price et al. / Chemical Physics 190 (1995) 123-130
assigned to a photon induced neutral-loss process, CF32+ + photon ~ CF 2÷ + F .
(5)
Fig. 2a also shows the presence o f f ÷ and CF~- ions following the interaction of the CF 2÷ ions with the laser radiation. Clearly these singly charged ions can be assigned to the products of photoinduced charge separation, CF~ ÷ + photon ~ CF~- + F ÷ .
(6)
Note also that very weak F ÷ and CF~- signals are observed in the background mass spectrum (Fig. 2b). These ions result from the continuous unimolecular dissociation of the metastable CF~ ÷ parent ions in the beam. It is interesting to observe that no unimolecular neutral-loss processes are observed, since no CF 2+ ions are detected in the background mass spectrum (Fig. 2b). It is also important to note the differences in peak shapes observed in the time of flight mass spectrum (Fig. 2a). The peak corresponding to the CF 2÷ ions is narrow, which indicates there is only a small energy release involved in the formation of this ion from CF~ ÷ . However, the F ÷ peak is wide, double peaked and asymmetric, and the CF~- peak is also wide. The width of these peaks indicates that the F ÷ and CF~ions are formed from CF32 ÷ with a considerable kinetic energy release. The energy release difference between the neutral-loss and charge separation channels is not surprising, since the forces involved in the neutral-loss and charge separation processes are markedly different. The observed width of the F ÷ peak corresponds to the formation of at least some of these ions from CF32÷ with an energy greater than 4 eV. The double-peaked shape of the F ÷ ion signal could arise from either the incomplete collection of these energetic ions or an anisotropic F ÷ velocity distribution generated by photoabsorption of the polarized laser radiation. However, we observe no significant change in the F ÷ peak shape as the orientation of the laser polarization vector is varied with respect to the ion beam direction. Hence, we consider the doubled and asymmetric F ÷ peak shape to result from the incomplete collection of the energetic ions by the TOFMS, a conclusion confirmed by simulations presented below. To explain the F ÷ peak shapes in detail requires consideration of the velocity components of the ions both parallel and perpendicular to the axis of the
TOFMS and the geometry of the TOFMS itself. The significant kinetic energy imparted to the F ÷ ions in the photoinduced charge separation can result in some F ÷ ions not reaching the detector, particularly when the resulting initial velocity vector has a significant component perpendicular to the axis of the TOFMS. Ions with significant velocity components perpendicular to the TOFMS axis would be detected at flight times close to the center of the flight time distribution for an ion of m/z = 19. Hence, the selective loss of these energetic ions results in the doubled peak shape. Additionally, the asymmetry of the F + peak is due to the optimization of the apparatus for collection of the doubly charged photoproducts; in this configuration an aperture (12 mm) at the entrance to the drift region stops some of the energetic F ÷ ions from reaching the detector (Fig. 1). The losses due to this restriction are more severe for ions formed with a significant impulse directed away (downward) from the TOFMS axis resulting in the asymmetric peak shape. The above qualitative arguments are borne out by detailed Monte Carlo simulations of the expected F ÷ peak shape. As shown by the solid line in Fig. 3, these simulations satisfactorily reproduce the experimental peak shape (bars) when the kinetic energy distribution of the F ÷ ions is modeled by a Gaussian distribution centered at an energy release of 4.5 eV with a full width
I
Illl
[IIz ~
8.99
9.98 Time of
flight /;~s
Fig. 3. Comparison of the experimental (error bars) and simulated (solid line) F + peak shape in the time of flight mass spectrum recorded following photodissociation of CF 2+ at 532 nm. The tail apparent on the peak to longer time of flight is attributed to the rise time of the ion extraction pulse, a factor not included in the simulation. See text for details.
S.D. Price et al. / Chemical Physics 190 (1995) 123-130
127
Table 1 Photofragments detected following the interaction of perfluorinated molecular dications with laser radiation Parent ion
CF] + SF 2÷ SF~ + SF 2 +
SF~ +
Photofragments detected
CF~ + F +, CF~ S 2+ F +, S ÷ SF 2+ F +, SF + SF 2 ÷ SF~ ÷ F +, SF~none
Photodissociation process
CF32+ ~ C F ~ + + F CF32÷ ~ C F ~ - + F ÷ SP e+ --->S 2÷ + F SF 2+ ~ S + + F + SF~ + ~ SF 2+ + F SF~ + ~ SF ÷ + F ÷ SF32÷ --->SF 2 + + 2F SF~ + ~ S F ~ ÷ + F SF~ + ~ S F ~ - + F ÷ -
Relative ion yield a 355 nm
532 nm
1064 nm
4.3 3.0 0.96 2.3 0.90 0.88 0.30 5.6 4.1 -
2.4 1.6 0.093 1.3 0.37 0.36 < 0.003 1.7 0.62
0.25 0.2 < 0.003 <0.012 0.027 0.040 < 0.003 0.33 <0.012
The photodissociation reactions producing the given photofragments are listed, as well as the relative ion yields of each photodissociation process. a As described in the text, the relative ion yields have been corrected for mass discrimination effects and the variation in the photon density as the wavelength of the laser is varied. The error limits for the ion yields are + 9 % for the neutral-loss processes and + 2 5 % for the charge separation reactions. The corrections for the singly charged ion signals for the losses in the apparatus introduce a significantly larger uncertainty in those values.
at half maximum (FWHM) of 2.5 eV. The 'doughnut' laser intensity distribution also contributes to the peak shape, and is modeled by restricting photodissociation events to a tubular shaped volume of inner radius 3 mm and outer radius 8 mm, oriented parallel to and centered on the laser propagation axis. The beam of parent dications is assumed to have a Gaussian intensity profile (FWHM of 5 mm) perpendicular to its direction of propagation. It is satisfying to note that under identical conditions the simulation also satisfactorily reproduces the CF~- peak shape using an ion energy distribution derived from the fitted F ÷ kinetic energy distribution by conservation of energy and momentum. These computer simulations also indicate that the F + peak shape should be sensitive to the position of the laser interaction region and the deflector setting, as we observe experimentally. The simulations show that with the experimental setup described above we detect = 20% of the F ÷ ions and = 3 5 % of the CF~- ions. Under identical conditions, formation of CF 2÷ with an energy release of = 100 meV results in collection of all the dications, with a narrow mass spectral peak, again as observed experimentally (Fig. 2a). Further experiments confirm that the photodissociation behavior observed for CF~ ÷ is common to other perfluorinated dications. In these experiments, we monitored the ionic products formed following the inter-
action of SF 2+, SF~ ÷ , SF~ ÷ , and SF~ ÷ with laser radiation at 355, 532, and 1064 nm. All the observed dication photodissociation processes, at three laser wavelengths, are listed in Table 1. For SF ÷ , SF~ + , and SF 2 + a behavior similar to that of CF~ ÷ is observed, with both neutral-loss and charge separation processes being induced by the laser photons. However, one notable difference is that SF~ ÷ can lose two fluorine atoms to form SF 2÷. No photoinduced reactions were observed for SF~+; presumably the first allowed absorption for this dication is at wavelengths shorter than 355 nm, the largest available photon energy. Table 1 also shows estimates of the relative ion yields for the different photodissociation channels for each dication. These values have been obtained from the mass spectral intensities with appropriate correction for all the discrimination effects of the apparatus as described above and the different photon densities at each laser wavelength. Hence, the relative magnitudes of these relative ion yields provide an estimate of the relative magnitudes of the photodissociation probability for different photodissociation channels. Due to the low dication currents generated for the sulfur species, and the fact that sulfurous deposits from the SF6 precursor gas rapidly contaminated the ion source, further investigations of the photodissociation phenomena were restricted to CF~ +. We studied the
128
S.D. Price et al. / Chemical Physics 190 (1995) 123-130
fluctuations in the laser power and ion beam intensity, over a laser wavelength range from 556 to 790 nm and with a linewidth of 1 cm- 1.
50 45 40" •~, 35
~
0
6
4
n
m
4. Discussion
~ " 3.0_J 25
20-
15 05
~
,.~
F* yield at 562nm
i .... 10
i .... 1.5
i .... 2.0
i 2.5
Log (Laser power) Fig. 4. Dependence of the yield of the photoinduced neutral-loss and charge separation processes ofCF~ + on laser power at 562 and 1064 nm. The slopes are 0.99 + 0.07 and 1.02-1-0.06 for CF~ + yield at 562 and 1064 nm, respectively. The slope f o r F ÷ yield at 562 nm is 0.98 + 0.15. The neutral-loss process was monitored by detecting CF~ + ions, while the charge separation channel was monitored by detecting F ÷ ions.
dependence on the laser power of the two photoinduced reactions (Eqs. (5) and (6)) for CF 2÷ . These experiments were performed by monitoring the photoproduct intensities while attenuating the laser beam with a wire mesh. As shown in Fig. 4, the power dependence of the ion yield confirms that multiphoton processes are not involved in the formation of either the CF 2+ or F ÷ photoproducts. We also recorded a dication photofragmentation spectrum by monitoring the photodissociation signal for both the neutral-loss and charge separation reactions ofCF~ + (Eqs. (5) and (6)) as the laser wavelength is varied. However, no spectral structure was observed in either the CF~ ÷ or F + signal, when corrected for
Recent work has shown that perfluorinated molecular dications exhibit a new form of collision-induced reactivity involving neutral-loss reactions [ 14], for example, CF 2+ + M - - * C F f + F + + M .
(7)
It is clear from the results presented above that photoinduced neutral-loss processes (Eq. (5)) also play an important role in the photochemistry of perfluorinated molecular dications. In the case of CF 2 +, since we observe no spectral structure (at the experimental resolution of = 1 c m - 1) in the photoinduced ion signal, it seems likely that both the neutral-loss and charge separation reactions result from bound--* free transitions. Indeed, similar transitions [2] have been observed for N 2+ and N O 2 + . In fact, both the photoinduced processes observed for CF~ + (Eqs. (5) and (6)) may arise from population of the same initial electronic state since, within the experimental error, the ratio of the ion yields of the two photochannels for CF 2+ (Table 2) does not vary with wavelength. This concept, that both the photoinduced neutral-loss and charge separation processes arise from the same excited electronic state, is consistent with a previously proposed model for the low-lying potential surfaces of perfluorinated molecular dications and CH3I 2+ [ 14,18 ]. This model was developed to explain
Table 2 Ratios of the relative ion yields for photoinduced neutral-loss and charge separation at three different laser wavelengths Parent ion
Neutral-loss to charge separation ratio 355 nm
CF3z+ SF 2 + SF~ + SF~ + "
532 nm
1064 nm
max
min
max
min
max
min
2.1 0.70 1.5 2.0
1.0 0.34 0.73 1.0
2.2 0.10 1.51 4.0
1.1 0.05 0.74 2.0
1.9 1.0 -
0.9 0.48 -
The m a x i m u m and m i n i m u m values of the ratios consistent with the experimental uncertainties in the relative ion yield (Table 1 ) are given. "The ion yield for formation of SF 2+ has been used to calculate these ratios.
S.D. Price et al. / Chemical Physics 190 (1995) 123-130
CF#*÷ F l E uJ
....
Et
CF2*+ F+ 1 r (CF2--FI Fig. 5. Schematic potential energy surfaces for a proposed electron transfer mechanism which allows competition of neutral-loss and charge separation from the same electronic state of CF~ ÷ following excitation. See text for details.
the competition between neutral-loss and charge separation reactions observed following collisional excitation of these species. As illustrated in Fig. 5, this model assumes that the initial excitation accesses a repulsive charge separating state, but as the ions separate there may be a curve crossing with an electronic state correlating with the neutral-loss asymptote. As has been discussed previously, the energetics of perfluorinated molecular dications may result in such a curve crossing at an interspecies separation where the coupling between these electronic states causes the neutral-loss and charge separating reactions to compete effectively [ 14]. Indeed, if this model is correct, it is tempting to conclude that the same electronic state of CF 2+ is populated in the photoexcitation and collisional excitation experiments. The curve crossing model explains why, in the absence of photoexcitation or collisional excitation, the only unimolecular decay pathway of the long-lived dications (e.g. CF 2+ ) in the ion beam involves charge separation (Fig. 2b). This is because, as is apparent from Fig. 5, the long-lived dication states will lie at energies below the curve crossing and, in the absence of additional excitation, they are unable to reach the asymptote corresponding to neutralloss. From the curve crossing model we can also predict that there exists an excitation threshold (Et in Fig. 5) below which the charge separation reaction should still occur but neutral-loss reactions would not be observed. As discussed below, this effect is observed for some of the SF] ÷ photodissociation reactions listed in Table 1. The study of the interaction of SF] + with laser radiation provides further indication that the same electronic states may determine the behavior of
129
perfluorinated dications in both collisional excitation and photoexcitation experiments. Collisional studies show that for SF42+ there is an activation energy in excess of 3 eV for the neutral-loss process [ 14]. This activation energy was tentatively interpreted as the excitation energy required to collisionally promote the ion to the electronic state responsible for dissociation (Fig. 5). Hence, if the same electronic states are responsible for both the photoinduced and collision induced reactivity, we would not expect SF42+ to exhibit any photoinduced reactivity at the photon energies employed in this study. This is indeed observed experimentally. The simple curve crossing model also is consistent with the observation of the photodissociation pathways f o r S F 2 + and SF 2+. For SFz2+, as for CF z +, the ratios of the ion yields of the neutral-loss and charge separation photodissociation reactions (Table 2) remain constant as the wavelength is varied. For SF z+ we observe (Table 1 ) that the threshold for both photodissociation channels lies between 532 and 1064 nm, and as the threshold is approached, the relative importance of the charge separation process increases (Table 2). As described above, such a trend is predicted by considering the energetics of the simple curve crossing model (Fig. 5). However, more subtle effects, such as the effect of the changing velocity through the crossing at lower excitation energies, could also enhance the nonadiabatic pathway. For SF 2 +, the data indicate that the threshold for the double neutral-loss reaction to form SF z + lies between 355 and 532 nm; whereas, single neutral-loss to form SF 2+ and charge separation are both observed at 532 nm. This indicates that at 355 nm at least two electronic states of SF~ + are accessed, one of which dissociates by double neutral loss to form SF 2 + and is inaccessible at 532 nm. The threshold for the charge separation reaction of SF~ + appears to lie between 532 and 1064 nm, whereas the threshold for single neutral-loss to form SF22+ appears to lie above 1064 nm. This threshold behavior is the opposite of expectation if both the charge separation reaction and the neutral-loss processes result from the decay of the same electronic state via the curve crossing mechanism. One explanation for this behavior is that the energetics of SF 2+ place the relevant curve crossings (Fig. 5) at interspecies separations where the coupling between the electronic states does not permit a competition between neutral-loss and
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S.D. Price et al. / Chemical Physics 190 (1995) 123-130
charge separation. In any case, it appears that for SF~ + there m a y be at least three different electronic states which can be populated by photoexcitation, and each electronic state dissociates to different products.
5. Conclusion W e observe several n e w photoinduced reactions of fluorinated molecular dications i n v o l v i n g neutral-loss to produce a daughter dication. These processes compete with photoinduced charge separation following absorption of a single photon of visible radiation. The results provide the first observation of photoabsorption by a polyatomic dication and the first observation of a photoinduced neutral-loss reaction of a dication.
Acknowledgement The authors gratefully acknowledge the Air Force Office of Scientific Research (High E n e r g y Density Matter Program) for their generous support of this research.
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