J. Phys. Chem. 1995, 99, 15438-15443
15438
Photodissociation and Rare Gas Reactions of CCb2+: Energetic Thresholds for the CCls Product Yin-Yu Lee and Stephen R. Leone*>: J I U , National Institute of Standards and Technology and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 Received: February 28, 1995; In Final Form: May 9, 1995@
This article reports the results of experiments to determine the energetic threshold for CC1+ production from CC13" by collisions with rare gases and by laser photofragmentation. An array of product channels is observed for the reactions of Cc13*+ with the rare gases at low ( < 100 eV) laboratory collision energy. By varying the ion translational energy with Ne as the collision partner, the signal of CC1+ increases dramatically at a centerof-mass collision energy of 7-9 eV. Photolysis of CC13*+ produces CC12*+, CC12+, and Cl' at 1064 nm; when the laser wavelength is tuned to shorter wavelengths, the fragment CCl+ occurs in the product mass spectrum as well. By surveying with a tunable laser wavelength, the threshold to create the CC1+ fragment 2 0.1 eV, suggesting that different excited by the photodissociative channel is a much different value, ~ 2 . f states are involved in the collisional and photodissociative processes.
Introduction Reactions between doubly charged ions and neutral molecules have been the subject of considerable experimental investigation in recent years.'-" Multiple product channels are observed for the reactions of many dications with rare gases at low kinetic collision energy.I2-l6 From the variety of product ions detected in the collision reactions, processes are assigned to ions formed by collision-induced charge separation (CICS), nondissociative and dissociative charge transfer (CT), and collision-induced neutral loss reactions (CINL), indicated in eqs 1-3, respectively.' l 9
'-
+ Rg - A- + B+ + Rg AB2+ + Rg - AB+ (or A+ + B) + RgABZc + Rg-A2+ + B + Rg AB2+
(1)
(2)
(3)
When a molecular ion absorbs a photon, a transition to an excited state of the ion is induced. If the excited electronic state dissociates in a time short compared to fluorescence, charged fragments as well as neutrals can be formed. The charged photofragments can be used as sensitive monitors of the photon absorption and subsequent decay process.'8-24 Rotationally resolved spectra of dications have only been reported for Nz2+ and N02+.25-34These investigations provide a detailed picture of the structure and dissociation dynamics of both dications. In recent studies, several polyatomic fluorinated molecular dications (e.g., CFs2+) were shown to dissociate by excitation in the visible region.'4 The studies found that dissociation occurs by single-photon absorption which promotes the CF3'+ molecule to unbound excited states. The experiments also derived apparent excitation thresholds for the photoinduced neutral-loss and charge-separation processes of SF*+ and SF3?+. These results revealed some of the first data on the stability, structure, and dynamics of doubly charged polyatomic molecular ions.
* To whom correspondence
should be addressed. Staff Member, Quantum Physics Division, National Institute of Standards and Technology. @Abstract published in Aduunce ACS Ab.wucts, October 1. 1995.
0022-3654/95/2099-1S438$09.00/0
In this paper, we report the results of experiments to investigate the interaction of the polyatomic dication CCl3" with rare gases and laser radiation. Several product channels are observed for the reactions of CC132+with rare gases at low laboratory collision energy (49 eV). Particularly, the relative yield of the CC1+ product strongly depends on the mass of the collision partner for these reactions. By increasing the centerof-mass (COM) collision energy, we observe that the signal of CCl+ increases dramatically at 7-9 eV when using Ne as a collision partner. The photochemistry of CC132+is also studied in the near-IR-UV range. Photolysis of Cc13*+ produces CC12?+ by neutral loss (eq 4) and CC12+ by charge separation (eq 5) in the near-IR-red region. By tuning the laser wavelength to shorter wavelengths, we find that the CCl' fragment occurs in the product mass spectrum as well. This photofragment correlates with the opening of a different photodissociation channel (see eq 6).
+ hv - CCl,,-,12+ + nC1 CC132++ hv - CC12&C1'
CC1,,2*
-
+ c1*+ cc1- + C1' + c1
CCl+
(4) (5)
(64 (6b)
The excitation threshold for the CCl+ production by the photoinduced dissociation reaction of CC13*+ (eq 6) is obtained from the product ion mass spectra by studies at several fixed laser wavelengths and also by tuning the output of a dye laser through the threshold. In the following sections we examine the product branching ratios for collisions of CC13*+ with all the rare gases and for photolysis of CC132+at various wavelengths. The thresholds and mechanisms of the reactions and dissociative electronic states are considered in detail.
Experimental Section The experimental apparatus used to study the collision reactions and photofragmentation of molecular dications has
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 42, 1995 15439
Photodissociation Reactions of CC132+
u -
c Filament
-. .-
Time-of-Flight Mass Spectrometer
Focus plate
#}
Flight Tube
Y
Ion Source
Spectrometer
4
Laser Plate Interaction Region
Ion Detector
Figure 1. Schematic diagram of the apparatus used to investigate the photodissociation reactions of molecular dications.
been described previously (Figure l).12-'6.24 Briefly, a massselected CC132+ dication beam is extracted from an electron impact ion source using a quadrupole mass spectrometer (QMS). The experiments are performed by intersecting the cc132+ dication beam with a pulsed beam of neutral rare gas atoms in the collision region. The same beam is crossed with a pulsed laser for the photolysis experiments. Product fragments are collected with a time-of-flight mass spectrometer (TOF-MS) for analysis. CC14 is used as the precursor gas for the generation of the CC132+ molecular dication. The ion source is held at.various voltages, which determines the kinetic energy of the dication beam. Ions are extracted from the source and focused into the QMS to reject all the ions formed in the ion source except the dication of interest. For the collision reactions, a continuous dication beam emerging from the QMS is focused into a collision region where it interacts with a pulsed jet of rare gas collision partner (not shown in Figure 1). The number density of rare gas atoms is carefully controlled to ensure that singlecollision conditions exist in this interaction region. The collision region is defined by a pair of electrically biased plates, and these bias potentials define an extraction field which sends the ions laterally toward the source region of the TOF-MS and a Faraday cup (see Figure 1). Both ion products and unreacted dications are extracted into the source region of the TOF-MS, which is used to detect and identify the ions. When the ion beam is not being sampled, the source region of the TOF-MS is maintained in a field-free state at 0 V potential. In this arrangement, the electrical potential of the ion source defines the transverse velocity of the dications. Following the interaction of the ion beam with a pulse of rare gas, the repeller plate of the TOF-MS is pulsed to a positive voltage, +300 V, to accelerate any ion products as well as the undissociated dications into the TOF-MS for detection and identification. A deflector arrangement near the start of the drift region (see Figure 1) is used to apply an electric field to compensate for the lateral velocity of the ions to steer them up the drift tube. Following the deflector, an einzel lens is employed to focus the ion packets onto the microchannel plate (MCP) detector at the end of drift region. The output from the MCP detector is preamplified and fed into a digital transient recorder. In a typical experiment, mass spectra are accumulated over 5000 coadditions of the time-of-flight data acquisition cycle, covering a mass range of approximately 4-165 amu. These results are summed using a signal averager and then downloaded to a microcomputer for analysis. For the photofragmentation experiments, we adapted the collision apparatus for interaction by a laser beam. The ions are generated by electron impact in the ion source from a pulsed beam of neutral precursor instead of the continuous nozzle to
improve the signal level and lifetime of the filament. This ion beam emerging from the QMS is focused and steered into the source region of the TOF-MS, where it is intersected perpendicularly by the laser radiation (see Figure 1). Two laser systems are used: (1) the fixed wavelengths of a Nd:YAG laser on the fundamental 1064 nm and harmonics, 532 and 355 nm, and (2) a tunable dye laser. The laser beam is typically 1.5 cm in diameter, providing a maximum energy density of 80 mJ cm-2 pulse-' at the highest laser energy of 18 1 mJ. Given the typical broad-band dissociative cross sections for small molecule this is sufficient to eliminate electronic states iO-i7-iO-20 multiphoton effects. Indeed, measurements reported below show linear power dependences down to less than 20 mJ pulse energy. Both the pulsed valve and the laser are operated at 10 Hz. In a typical photodissociation experiment, mass spectra are accumulated over 2000-5000 coadditions of the time-offlight data acquisition cycle, also covering a mass range of approximately 4-165 amu. The results are summed using a signal averager and then downloaded to a microcomputer for analysis. All the results for the photodissociation experiments reported here are at a constant source ion kinetic energy of 49 eV,I2 with deflector voltages of and -1 15 V, which is near the optimum for the collection of the CCl+ ion. Product ion mass spectra are recorded both before and after the rare gas jet, or the laser radiation, is pulsed. The spectrum recorded before the rare gas jet or laser radiation pulse is a background spectrum that contains signals from both unimolecular decay processes and collisions with background gases. These signals are subtracted from the spectra recorded after interaction with the rare gas jet or laser radiation to obtain the product ion signals resulting solely from the specified reaction. The ion collection efficiency of the TOF-MS is dependent upon the voltages applied to the deflector plates.12.13 The net product ion signals are corrected for these mass discrimination effects introduced by the deflectors to obtain the product ion yields for a given reacting system. Figure 2 shows the variation in the product ion signal measured relative to the deflector voltage at a fixed dication kinetic energy of 49 f 1 eV for the collisions of CC132+ with Ar. Relative yields for the product ions of the reactions with rare gases are measured with deflector voltages of and -125 V applied symmetrically across the TOF-MS deflectors, which is near the optimum for the collection of all ions. The 2450 V drift voltage is applied constantly to the TOFMS. Kinetic energy dependence experiments are carried out on the CCl+ product by changing the dication potential in the ion source region. The laboratory frame kinetic energy is defined by the extraction plates in the collision region with an accuracy of f1 eV for the dication.I2 The deflector voltages of the TOFMS are kept at a constant value of and -1 15 V, which is
+
+
+
15440 J. Phys. Chem., Vol. 99, No. 42, 1995
"'II
cc1,2+
CCI' \
CI' I
CCI,' /
Lee and Leone
I
CCI,*
0 -Hep_ 2kJL.-., -_ 50
100
150
200
1 ' ~ " " " ' l ' ' ~ ' 1 " ' ~ " " ' 1 " " I " ' r T ' " ' l " ' I " ' ' I " ' ' l " ' '
250
Deflector Voltage (V)
Figure 2. Variation in the product ion signal measured with respect to and -125 V symmetnc deflector voltages for Cl', CC12?+,CCI', CC12', and CCI'' ions entering the TOF-MS after collisions of CCI?' with Ar. Each signal is normalized here to 100, only for ease of display.
+
near the optimum voltage for the collection of the CC1+ at a CC13?+ kinetic energy of 49 eV. Therefore, the collection efficiency for the different kinetic energies of the source ions is corrected experimentally by using relative yields from the TOFMS spectra for each source voltage setting. For a given source ion voltage, we measure the variation of product ion signal with respect to the deflector voltage on the TOF-MS and make a correlation diagram individually, as shown in Figure 2 for a source dication kinetic energy of 49 eV. These results are used to normalize the ion signals for the deflector collection error.
"A
,111
Results Collision Reactions. The collision experiments are conducted at 49 eV collision energy in the laboratory frame for the reactions of CCl3?+ with each of the rare gases. To determine the relative product ion yield for each dication reaction, 3-5 time-of-flight mass spectra are recorded at an 830 ps delay following the voltage pulse applied to the pulsed valve. Background mass spectra are also gathered by pulsing the TOFMS repeller plate 300 ps before the pulsed valve is opened to detect any background ions present due to unimolecular decay of the dication, leakage of stray ions through the QMS, or reactions of the dication with any residual gases in the chamber. The ion signals from all the mass spectra are determined by the integration of the mass peaks. Figure 3 shows TOF mass spectra for the reactions of CC13?' with rare gases. The relative yields of the product ions from the reactions of CC132+ with each rare gas after corrections for detection efficiencies are shown graphically in Figure 4. The product mass spectrum for the reaction of CC1j2+ with He consists mainly of C1+, CC12'+, and CC12+. The CC1z2+ ion must be produced via a collision-induced neutral loss chanr1e1.I~No CC13+ signal is observed in the product mass spectrum, suggesting that the charge transfer channel is not yet energetically accessible in this system. Since the C1+ and CC12' ion yields (Figure 4) are approximately the same magnitude, this product ion pair is most likely created by a collision-induced charge separation process. The main products from the reaction between CC13?+ and Ne are the same as the products in the CC132+ He system. The neutral loss product contributes more than 40% of the product ions. A slight amount of CCl+ and CCl3' is observed from the reaction of CC13*+with Ne. The CCl3+ ion must derive from the charge transfer channel. According to eqs 1 and 2, the CC12+ ion can be created by either the CICS or dissociative
+
"I
He
Ne
0CI*
B CC1,2'
Ar I CCI'
Kr E2
CCI,'
Xe
E3 CCI;
Figure 4. Relative product ion yields following collisions of CC13?+ with the rare gases at a laboratory collision energy of 49 eV. The error bars represent one standard deviation of the data from all runs.
charge transfer (DCT) process. Since the C1' and CC12+ ions are formed in unequal numbers (the yield ratio of CC12+/C1+ is 2.3), the dissociative charge transfer processes must occur as well as the CINL and CICS reaction channels in this system. For the reaction CC132+ Ar, again, the major products are mainly C1+, CC12+, and CC12+; the CClzZf ion produced by neutral loss is a strong component of the product ions. The CCl+ ion is an obvious product, and a small amount of the CCl3+ ion appears in the product mass spectrum. The yield of the CC12' ion is twice the yield of the C1+ ion. The main products of the CC13*+ Kr reaction are the total charge transfer species, C1+, CCl', CC12+, and cc13'. Particularly, the CC13+ ion dominates the product ions of this reaction, and the yield ratio for CC12+/Cl+ is 2.9 f 0.5. Thus, nondissociative and dissociative charge transfer processes predominate in this system. The neutral loss product (CCl?>+)yield is a small fraction of the total ions (see Figure 4). In the reaction of CC13*+ with Xe, the dissociative charge transfer products dominate the product ions, and CC12+ is the major product of this reaction (70%). This suggests that dissociative charge transfer occurs more readily because of the larger exothermicity of the reaction. The contribution of the neutral loss product shrinks to less than 3% of the total product ions in this system. From an overview of the products of the collision reactions of Cc13** with the rare gases, we can conclude the following: (1) The increased ionic signal observed in moving from the product mass spectrum for CC112+-He collisions to CC13"-
+
+
J. Phys. Chem., Vol. 99, No. 42, 1995 15441
Photodissociation Reactions of CCls2+ I
I
l
0
5
'
"
'
l
10
'
-
"
l
'
15
~
'
~
20
COM Kinetic Energy (eV) Figure 5. Center-of-masskinetic energy dependence of CCI+ ion signal from the collision of CC13*+ with Ne.
Xe implies an increased collision reaction cross section for the heavier rare gases. (2) The amount of CC12+ product ions surpasses the amount of Cl+ product ions in each of the collision reaction systems except the reaction with He, in which the CC12+ and C1+ ions are produced in approximately equal amounts. The differences in the yields of CC12+ and C1' in each reaction system become more prominent when CC132f reacts with the heavier rare gases. The product yield ratio of CClz+/Cl+ in each reaction system is 2.3 f 0.4,2.0 f 0.3, 2.9 f 0.5, and 28 f 5 for collisions of CC132f with Ne, Ar, Kr, and Xe, respectively. Assuming that the C1+ ions are totally produced by CICS, this would suggest that both DCT and CICS channels occur, and DCT surpasses CICS when the heavier rare gases, Kr and Xe, are used as collision partners. (3) The doubly charged product yield, CCh2+ (collision-induced neutral loss product), is the largest for the systems in which charge transfer does not compete effectively with the collision-induced processes. (4) A small amount of CC1+ product ion appears in the product ion mass spectrum of the collision reaction of CCL2+ with Ne at a laboratory kinetic energy of 49 eV. There is an observed increase in this ion signal when moving from the product mass spectrum for CC132+-Ne collisions to the product mass spectrum for CC132f-Xe. This indicates that CCl+ correlates with the opening of a new charge separation process or a dissociative charge transfer process which is driven by the available exothermicity. This threshold is found to lie around a center-of-mass kinetic energy of 8 eV, as discussed below. By changing the parent dication velocity, we conducted collision energy dependence experiments of the CC132+dication with Ne. The product ion mass spectra are recorded following interaction of the beam of CC132+ ions with Ne. We observe that both neutral loss and charge separation processes are activated over the range of COM kinetic energies from 2.7 to 17.1 eV. The CC122+ product yield is constant at 18 f 3%. Figure 5 shows the dependence of the CC1+ product ion signal on collision energy for Cc13*+ with Ne. The product ion CCl+ signals exhibit a threshold phenomenon with a first appearance at 7 eV. A linear extrapolation of the higher energy data would suggest a threshold of 8 eV or larger. Laser Photofragmentation. Figure 6 shows the product ion mass spectra recorded following the interaction of a beam of cc132+ dications with the Nd:YAG laser harmonic wavelengths of 1064, 532, and 355 nm in traces b, c, and d, respectively. Trace a shows a mass spectrum of a beam of CC132+ ions recorded without any laser radiation. These traces are normalized by the photon flux but not corrected with respect to the mass discrimination effect of the deflector. The formation of
Figure 6. Time-of-flight mass spectra recorded following the interaction of a CC132+beam with Nd:YAG laser fundamental and harmonic radiations. The laser wavelengths are 1064, 532, and 355 nm respectively for traces b, c, and d. Trace a shows a mass spectrum with the laser radiation blocked. The deflectors are set at and -1 15 V. The origins have been displaced vertically by 1 x lo4, 2 x 104, and 3 x lo4 counts respectively for traces b-d.
+
CC1Z2+from CC132+ can be definitively assigned to a photoninduced neutral loss process, eq 4.24 CC132f
+ hv - CC122f + C1
Traces b-d in Figure 6 show the presence of C1+ and CC12+ ions following the interaction of CC132+ions with the Nd:YAG fundamentaland the two harmonics. After correctionsare made, the Cl+ and CC12+ ion yields are formed in equal amounts, within one standard deviation of the mean errors, for the interaction of CCb2+ with laser radiation at 1064 nm. This singly charged product pair can be assigned to the products of the photoinduced charge separation process, eq 5 .
+
-
+
C C ~ ~ hv ~ + C C ~ ~ +CI+ For the photoinduced processes, the ratios of the relative ion yields for neutral loss and charge separation are 0.27 & 0.04, 0.32 & 0.05, and 0.29 f 0.04 for photolysis of CC132+by 1064, 532, and 355 nm, respectively. In traces c and d, in addition to the Cl+, CC1z2+,and CC13+ production, CC1+ signals are distinctly observed in the mass spectra, corresponding to the interactions of laser radiation at 532 and 355 nm or photon energies of 2.33 and 3.49 eV, respectively. The CC1+ signal is more intense for 355 nm than for 532 nm. In addition, the C1+ ion yield is in excess of the CC12+ product by 10% at 355 and 532 nm, and a small amount of C12+ also appears in the product mass spectra, traces c and d. Both CCl+ and C12+ are not observed in the product mass spectrum of the photofragmentation of CC132+ with 1064 nm radiation. The observed product species at the higher photon energies suggest that the CCl' product is produced by both of the photoinduced charge separation processes of eq 6. By scanning the tunable dye laser to shorter wavelengths (higher photon energy), we find that the CCl+ product channel is opened at a threshold of 2.2 0.1 eV. Note that differences in peak shapes are observed in the TOFMS spectra (Figure 6b-d). The neutral loss product CCh2+ ion peak is narrow, which indicates only a small energy release involved in the formation of this ion from CCls2+,whereas the peaks corresponding to the charge-separated products are broadened. The C1+ peak is wide and triple peaked, and the CC12+ peak is also wide. The broadening of these peaks implies that ions are created from cCls2+ with a considerable kinetic
*
15442 J. Phys. Chem., Vol. 99, No. 42, 1995 I o5
Lee and Leone
CI'
2
i
\
r 10
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1
1
I
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" ' " ' " '
200
Power (mJ) Figure 7. Power dependence of the photofragment yields of the photolysis of CC13?+at the wavelength of 532 nm. The slopes are 0.96 0.05, 1.03 0.08, and 1.08 0.15 for the C1' (0),CCI? (U), and CCI' (*) product ions, respectively. The deflectors are set
*
at
+ and -115
V.
energy release. The energy releases are different in the neutralloss and charge separation channels, because the forces involved in the excited state and the separated limits of energy are different. By measurement of the peak separation, for the 532 nm data, the additional kinetic energy of the C1+ ions is estimated2j to be 2.4 f 0.2 eV. However, we do not attempt any additional analysis of the kinetic energy release data. To verify that photoproduct ions are not produced by multiphoton processes, we investigated the dependence of the laser power on the photoinduced product ions. These experiments were performed by monitoring the product ion while attenuating the laser beam with neutral density filters or metal wire meshes. The CS and NL processes were monitored by detecting the C1+ and CC12?+ ions, respectively. Figure 7 shows the power dependence of the ion yield, which confirms that multiphoton processes are not involved in the formation of the C1+, CC12?+,or CCl+ photoproducts.
Discussion Collision Reactions of CCb2+ with Rare Gases. The collision results presented above show that multiple product channels are observed for the reactions of CClS2+ with each rare gas species at a laboratory collision energy of 49 i 1 eV. Recent studies have shown that perfluorinated molecular dications exhibit collision-induced reactions involving both chargeseparation and neutral-loss processes. I 4 - l 6 It is evident from the results shown above that similar collision-induced reactivities also occur in this perchlorinated molecular dication, CC132+,at a low laboratory frame kinetic energy of 49 eV. The increased signal of CINL on moving from He to Ar (see Figure 4) is likely due to the increased center-of-mass kinetic energy available for dication excitation as well as the larger size and polarizability of the neutral atom. For He, the charge transfer channels are not yet energetically accessible, and hence there can be no competition between CT and CINL. When both the center-of-mass kinetic energy and the collision cross section increase, an increase in the CINL signal results. Although DCT (eq 8) is energetically less favorable than CICS (eq 7 ) ,by 8.6 eV35when using Ne as a collision partner, we observe a large contribution of the CC12+ product from the DCT channel (eq 8). CC13'+
+ Ne - CC12+ + C1' + Ne
-
(7)
CC1,+ C1+ Ne+ (8) Excess CC12+ signal compared to C1+ or CCl3+ in the reaction
(CCI, -
ci)
Figure 8. Schematic potential energy surfaces for a proposed curve crossing electron transfer mechanism which allows competition between neutral loss and charge separation from the same electronic state of CcI?' following excitation.
of CC132+with collisions from Ne to Xe indicates that charge transfer followed by dissociation to CC12+ C1 is a process competitive with the sum of CICS and CT channels. The charge transfer CCI3+ product is very weak in the product mass spectra of the CC132+ Ne and Ar systems. Probably, these reactions access a dissociative state of CC13+ in the energy region where it is populated, leading to CC12+ C1 products (and removing the CC13' signal). On the other hand, with Kr or Xe, even though more kinetic energy and exothermicity are available to reach higher metastable states, a stable CC13+ product is produced, possibly involving electronically excited product states. The competition between CINL and CICS processes and the ratio of the product ions from these two processes differ for various dication-rare gas collision systems. Here, except for the CC1+ ion, the ratio of the ion yields for the neutral loss and charge separation channels does not vary with laser wavelength or collision energy within the experimental error. These results can be interpreted by applying a proposed model which has been used successfully for the low-lying potential surfaces of perfluorinated and CH@+ molecular dications.15,24.36.37 Following an interaction with a photon or energetic collision, the dication can overcome the endothermicity to NL products or over a barrier to CS products. The barrier to NL could correspond to the vibrational excitation required to break a C-C1 bond or to the energy required to reach a dissociative electronic state. The occurrence of a NL reaction could indicate a weak CC122+-Cl bond. which is easily broken following dication excitation. Thus, one could picture CC13?+ as being made up of a strongly bound dication core bonded weakly to a chlorine atom. As illustrated in Figure 8, this model assumes that the initial excitation accesses a repulsive charge separating state; then there may be a curve crossing to the neutral loss potential energy surface as the ions separate. (The interaction at the avoided crossing, determined by coupling matrix elements and velocity, must be between the adiabatic and diabatic limits in order for there to be an effective competition between NL and CS.) The relative energies of the CS and NL asymptotes determine whether the curve crossing lies at an appropriate internuclear distance to allow the competition between NL and CS. Therefore, the NL and CS product ratio varies with collision partner as a consequence of the energetics of the dication system. When the initial excitation energy exceeds both the NL and CS asymptotes, the effect of the changing kinetic or photon energy in a given reaction system would not significantly change the ratio of CS and NL products, as in the case of CC13'+. However, if the initial activation energy is between the CS and NL asymptotes, the excitation could
+
+
+
J. Phys. Chem., Vol. 99, No. 42, 1995 15443
Photodissociation Reactions of CC132t enhance the adiabatic (dotted) pathway in Figure 8. Therefore, we can predict the existence of an excitation threshold as well. This concept is realized by either increasing the COM kinetic energy in the collision reaction of Cc13*+ Ne or changing the photon energy in the photofragmentation of CC132+. Energetic Thresholds of CCl+ Ion Production. The CC1+ ion can be created in several collisional processes in the reaction of CCb2+ ions with Ne, summarized by two types of reaction processes, CICS (eqs 9a and 9b) and CT (eqs 10a and lob),
+
CC1;'
+ Ne - CC1' + C1,' + Ne -CC1' + C1+ + C1+ Ne -CC1+ + C1, + Ne'
-
CC1' +2C1+ Nef
(9a) (9b) (loa) (lob)
The dissociation energies of C12 and Cl2' have been measured to be 3.99 and 2.475 eV, respectively. The ionization potential energies of the C1, Cl2, and Ne have been determined acc~rately.~ These ~ values allow us to calculate the relative energies of these reaction processes. Of the CCl+ product channels, eq 9a is the lowest energetic channel for the production of the CC1+ ion. From the ionization potential energies of C1 and Ne and the Clzf dissociation energy, the asymptotic states for eqs 9b, loa, and lob will be 3.99, 10.08, and 12.55 eV above the dissociation asymptote of eq 9a, respectively. The CC1+ photoproduct threshold is estimated experimentally to be 2.2 & 0.1 eV. From the possible photodissociation channels below, CC132'
+ hv - CC1+ + C1; - CC1+ + C1' + C1
(6b)
eq 6a is the lower lying pathway to create the CC1+ ion. It is the best candidate pathway for CC1+ production. In contrast, Figure 5 shows that there exists a CCl+ product threshold at 7-9 eV COM kinetic energy in the collision reaction of CC132+ with Ne. We speculate that the charge separation process leading to the dissociative channel, eq 9b, is involved in this threshold phenomenon observed in the collision reaction (Figure 5). The cross section of eq 9a is most likely too small to be sensed in these experimental conditions, perhaps for FranckCondon geometry reasons. In summary, the present results give direct evidence for differing pathways between photoabsorption and reaction of a polyatomic dication and provide determinations of the energetic thresholds for these processes. The dication reaction chemistry and photodissociation dynamics provide rich new details on the electronic structure of these metastable species. Additional theoretical work will facilitate a deeper understanding of these processes.
Acknowledgment. We gratefully acknowledge extremely valuable discussions with S . D. Price and financial support from the Air Force Office of Scientific Research.
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