6412
J . Phys. Chem. 1991, 95, 6412-6415
Evidence of Charge Inversion in the Reaction of Singly Charged Anions with Multiply Charged Macroions Rachel R. Ogorzalek Loo, Harold R. Udseth, and Richard D. Smith* Chemical Sciences Department, Pacific Northwest Laboratory, Richland, Washington 99352 (Received: May 14, 1991; In Final Form: June 21, 1991)
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Novel gas-phase reactions were studied by mass spectrometryusing merged gas streams containing oppositelycharged species at near atmospheric pressure. Protonated adenosine 5’-monophosphate(AMP) and fluorescein molecules (M + H)+are observed as products in the reaction of multiply charged macroions (M’ + nH)“+ with species generated in the negative electrospray ionization of AMP or fluorescein. The (M + H)+products most likely arise from charge inversion of (M H)-anions in an ion-ion reaction. Protonated fluorescein and AMP were observed as a product of reaction with multiply protonated myoglobin, cytochrome c, and melittin but not with the less highly charged serine and bradykinin ions or other positively charged species of lower proton affinity from an air discharge. These observations suggest that the (M + H)+ products arise from charge inversion of (M - H)-anions in a single-stepencounter of the negative ion with a multiply charged cation.
Introduction Electrospray ionization (ESI) is providing mass spectrometry with new capabilities in molecular weight and structure determination.l-s Other endeavors are also poised to benefit from the advanced capabilities1v2in efficiently producing gas-phase ions from solutions of nonvolatile species. In particular, ion-molecule6 and ion-ion reaction studies now have easier access to multiply charged ions and macroions. In this initial communication on ion-ion reactions based upon these methods, we report the observation of protonated adenosine 5’-monophosphate (AMP) and fluorescein molecules, (M H)+, as products in the reaction of highly charged macroions, (M’ nH)”+,with deprotonated species generated in the negative ESI of AMP and of fluorescein. We believe that the (M + H)+products arise from charge inversion of (M - H)-anions in a novel gas-phase ion-ion reaction involving transfer of two protons. These reactions are indicative of a rich class of gas-phase processes that previously could only be studied indirectly in the condensed phase.
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Experimental Section The ESI source and the mass spectrometer employed here differ from those of our previous ~ o r k 4 , ” ’in~ that the nozzle/skimmer inlet9J0 was replaced by a new design, shown schematically in Figure 1, the details of which will be provided elsewhere.” The capillary inlet is a variation of the interface developed by Whitehouse et a1.,I2but it is fabricated in the shape of a Y,providing multiple paths for merging streams of charged species formed from spatially separated ion sources. Our approach differs from the merged beam reactors employed in mutual neutralization studies of small ionsI3J4in that the region where the ion streams intersect is at near atmospheric pressure, m / z selection of the reactants is not obtained, and the reactions have been studied at modest temperatures (typically 300-500 K). The reactant ions are generated by dual ESI sources8 or alternatively by an ESI source and an atmospheric pressure discharge in air. Ion desolvation prior to the sampling orifice (inlet) was accomplished either with the aid of a warm, countercurrent flow of N2(350 K) and/or by heating the capillary with an auxiliary heating element. If desolvation is accomplished solely by capillary heating, larger amounts of solvent (generally water, methanol, and a smaller amount of acetic acid) enter the capillary. The capillary outlet voltage could be biased relative to the beam skimmer (held at ground potential) to further desolvate analyte ions through collisional processes or, at higher voltages, to collisionally dissociate them.”’ The capillary inlets and outlet were electrically connected to maintain equipotential across the capillary. *Towhom correspondence should be addressed.
Results and Discussion Electraspray ionization can efficiently transfer ions from solution to the gas phase by a process that, at the macroscopic level, involves (1) (a) Dole, M.;Mack, L. L.; Hines, R. L.; Mobley, R. C.; Ferguson, L. D.; Alice, M. B. J. Chem. Phys. 1968, 49, 2240-2244. (b) Mack, L. L.; Kralik, P.; Rheude, A.; Dole, M.J. Chem. Phys. 1970, 52, 4977-4986. (2) (a) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S.F.; Whitehow, C. M. Science 1989,246,64-71. (b) Fenn, J. B.; Mann, M.; Men& C. K.; Won& S.F.; Whitehow, C. M. Mass Specfrom.Reu. 1990, 9, 37-70. (c) Mann, M. Org. Mass Specrrom. 1990,25, 575-587. (3) Huang, E. C.; Wachs, T.; Conboy, J. J.; Henion, J. D. Anal. Chem. 1990,62, 713A-725A. (4) (a) Smith, R. D.; Loo, J. A.; Barinaga, C. J.; Edmonds,C. G.; Udscth, H. R. J. Am. Soc. Mass Spec?rom. 1990, 1, 53-65. (b) Smith, R. D.; Loo, J. A,; Edmonds,C. G.; Barinaga, C. J.; Udscth, H. R. A M / .Chem. 1990,62, 882-899. (c) Smith, R. D.; Barinaga, C. J. Rapid Commun. Mass Spectrom. 1990, 4, 54-57. (d) Loo, J. A,; Ogorzalek Loo, R. R.; Udscth, H. R.; Edmonds, C. G.; Smith, R. D. Rapid Commun. Mass Spectrom. 1991, 5, 101-105. (e) Edmonds, C. G.; Loo, J. A.; Barinaga, C. J.; Udscth, H. R.; Smith, R. D. J . Chromatogr. 1989, 474, 21-37. (f) The sodium salt was sprayed in ref 4c, while the free acid was sprayed in this work. ( 5 ) (a) Chowdhury, S.K.; Katta, V.; Chait, B. T. J. Am. Chem. Soc. 1990, 112,9012-9013. (b) Katta, V.; Chait, B. T. Rapid Commun. Mass Spec?rom. 1991, 5, 214-217. (c) Katta, V.; Chowdhury, S. K.; Chait, B. T. J. Am. Chem. Soc. 1990, 112, 5348-5349. (d) Jayaweera, P.; Blades, A. T.; Ikonomou,M.G.; Kebarle, P.J. Am. Chem. Soc. 1990,112,2452-2454. (e) Blades, A. T.; Jayaweera, P.; Ikonomou, M.G.; Kebarle, P.J. Chem. Phys. 1990, 92, 5900-5906. (f) Blades, A. T.; Jayaweera, P.;Ikonomou, M.0.; Kebarle, P.In?.J. Mass Specrrom. Ion Proc. 1990,101,325-336. (g) Ikonomou, M. G.; Blades, A. T.; Kebarle, P. A M / . Chem. 1990,62,957-967. (h) Bruins, A. P.Mass Spectrom. Reo. 1991, 10, 53-77. (6) (a) McLuckey, S.A,; Van Berkel, G. J.; Glish, G. L. J. Am. Chem. Soc. 1990,112,5668-5670. McLuckey, S.A.; Van Bcrkel, G. J.; Glish, G. L. Proc. 38th ASMS Conference;Tucwn; AZ, pp 1134-1 135; (b) Blades, A. T.; Jayaweera, P.;Ikonomou, M. G.; Kebarle. P.In?.J. Mass Specrrom. Ion Proc. 1990, 102, 251-267. (7) Loo, J. A.; Udseth, H. R.; Smith, R. D. Rapid Commun. Mass Specrrom. 1988, 2, 207-210. (8) Barinap, C. J.; Edmonds, C. G.; Udseth, H. R.; Smith, R. D. Rapid Commun. Mass Swcrrom. 1989.3. 160-164. (9) Smith, R. D.; Olivares, J.’ A.; Nguyen, N. T.; Udscth, H. R. AM/. Chem. 1988, 60,436-441. (IO) Smith, R.D.;Barinaga, C. J.; Udseth, H.R.Anal. Chem. 1988.60, 1948-1 - - .- - 952 - - -.
(11) Ogorzalek Loo, R. R.; Udseth, H. R.; Smith, R. D., manuscript in preparation. (12) Whitehouse, C. M.; Dreyer, R. N.; Yamashita, M.;Fenn, J. B. AMI. Chem. 1985, 57,675-679. (13) (a) Aberth,W.;Peterson, J. R.; Lorents, D. C.; Cook, C. J. Phys. Reu. Lerr. 1968,20,979-981. (b) Olson, R. E.;Peterson, J. R.; Moseley, J. T. J. Chem. Phvs. 1970. 53. 3391-3397. IC) Moselev. J. T.: Aberth. W. H.: Peterson, j. R. Phis. keu. Let?. 1970,’ 24, 435439. (dj Aberth; W. H.f Peterson, J. R. Phys. Reo. A 1970,1, 158-165. (e) Peterson, J. R.;Aberth, W.H.; Moseley, J. T.; Sheridan, J. R. Phys. Reu. A 1971, 3, 1651-1657. (14) (a) Neynaber. R. H. Adu. At. Mol. Phys. 1969,5,57-108. (b) Olson, R. E. J . Chem. Phys. 1972,56,2979-2984. (c) Burdett. N. A.; Hayhurst, A. N. Chem. Phys. Le??.1977,48,95-99. (d) Burdett, N. A.; Hayhurst, A. N. J . Chem. Soc., Faraday Trans. I 1978, 74, 63-70. (e) Burdett, N.A.; Hayhurst, A. N.J. Chem. Soc., Faraday Trans. I 1976, 72, 245-256. (f) Smith, D.; Church, M.J. Inr. J. MassSpecrrom. Ion Phys. 1976, 19, 185-200.
0022-3654 I9 112095-64 12S02.50 ~,IO 0 1991 American Chemical Society I
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The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6413
Letters
j
Ion
Lens RF Cusdruwb
E I e c I f o ~ p f 1~ ~
I
Sklmmer
\
auadrupolr Mass Fillet
Conversion Dynods
I
Electron MuIIipIter
TO Dala System
Electrospray 2
t
cry0
Roots Blower
Pumo
23+22+
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,B
100
X8
(M+24H)24+
Turbo Pump
25+
Figure 1. Schematic of the Y-tube dual electrospray apparatus and the mass spectrometer used for these studies. 0
TABLE I
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Downloaded by NEW YORK UNIV on September 13, 2015 | http://pubs.acs.org Publication Date: August 1, 1991 | doi: 10.1021/j100170a006
aoo
300
cat ionic charge reactant (M') M/ charging* inversion' Production of (M H)+ from Adenosine 5'-Monophosphate Anions myoglobin 16951 +11 to +25 (20) yes 12360 +IO to +I9 (15) yes cytochrome c melittin 2486 +3 to +6 (4) Yes 1060 +1 to +3 (2) no bradykinin 105 +1 no serine 126 +1 no thymine pyridine 79 +I no 101 +I no triethylamine 5% acetic acid/H20 +1 no +I no positive discharge in air, H(H20)"+
100 h
.-*
B
4
I
TB (M+H)+ 348
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Production of (M H)+ from Fluorescein Anions myoglobin 16951 +11 to +25 (20) yes 12360 +IO to +19 (IS) yes cytochrome c melittin 2846 +3 to +6 (4) weak 1060 +1 to +3 (2) no bradykinin 105 +I no serine 5% acetic acid/H20 +I no +I no positive discharge in air, H(H20),,+ oMolecular weight of neutral species ionized in the positive ion electrospray. bExtent of charging observed for M' species, and dominant charge state for molecular species from positive ion electrospray. Cyes = observed; no = not observed; weak = small signal intensity detected, but much less abundant than for myoglobin and cytochrome c.
nebulization of highly charged droplets followed by their evaporation.'+ Negative ESI-MS of AMP (structure I) in H 2 0 (with NH2 I
360
500
400
600
700
E )O
mlz FIgure 2. Top: positive ion spectrum obtained from positive ESI of horse heart myoglobin (M', hf, 16951). B = background (OS = 560 V). Bottom: positive ion spectrum obtained from positive ESI of horse heart myoglobin and negative ESI of AMP (M, hf, 347). The m / z 268 ion, labeled with an asterisk, corresponds to loss of HPOl from AMP (M +
H)+.Both spectra are plotted on the same absolute intensity scale. The inset spectra were obtained in separate scans with 8 times higher gain.
assignment as a weakly bound adduct. It is likely that (M + 0 A C ) - is the collisionally stabilized product of anion-molecule reaction of AMP (M - H)- with acetic acid evaporated from electrospray droplets and clusters.] Positive ion detection yielded the well-known myoglobin spectrum"b*4"and an additional ion at m / z 348, present only when both ion sources were operating. (See Figure 2.) This ion, assigned as the AMP (M H)+cation, was also produced when other multiply protonated macromolecules were substituted for myoglobin in the positive ESI source but not when a similar 5% acetic acid/water solution without myoglobin was sprayed or when the pasitive ESI source was replaced by a positive discharge source in air. The latter two systems are sources of singly charged ions of relatively low proton affinity compared to AMP, such as (H20),H+ (when n is low),l5 and that generate comparable ion currenrs. We note also that since current introduced from the ESI source into the capillary was nearly independent of cation identity, cation concentrationsare lower for more highly charged species. The charge inversion results are summarized in Table I. When the AMP (M - H)- intensity is monitored, there appears to be little difference between dual-spray and single-spray operation, indicating only a small extent of anion reaction. Also apparent in Figure 2 is an m / z 268 fragment ion of AMP, corresponding to the loss of HP03, probably from dissociation of (M + H)+ in the OS region.
+
I OH
I OH
adenosine 5'-monophosphale (I) a methanol sheath solvent) yielded primarily (M - H)- at m / z 346, as expected from previous A second, oppositely biased ESI source was used to spray a solution of horse heart myoglobin in 5% acetic acid/water (with a methanol sheath solvent) yielding a positive ion spectrum dominated by a distribution of multiply protonated myoglobin molecules. Upon merging the flowing streams of oppositely charged ions (i.e., operating both sources) the negative ion spectrum was qualitatively unchanged except for the addition of a peak at m / z 406, attributed to the acetate adduct, (M + 0AC)-. [The m / z 406 ion signal was approximately 50% as intense as (M - H)-at low outletskimmer (OS)voltage differences and in the absence of countercurrentN2, but was not observed at higher OS voltages, consistent with its
(15) (a) Dzidic, 1.; Carroll, D. I.; Stillwell, R. N.;Homing, M. G.; Horrung, E.C. Adv. Massspcrrom. 1978,7,319. (b) Lane,D. A.;Thomson, B. A.; Lovett, A. M.;Reid, N.M.Adv. Mass Spctrom. 1980,8. 1480. (c) Harrison, A. G. Chemical Ionization Mass Spectrometry;CRC Press: Boa Raton, FL, 1984, 52, 80.
6414 The Journal of Physical Chemistry, Vol. 95, No. 17, 1991
22+
B
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B B
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(M’+23H)23+
E’”
1 ll
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0 800
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It is well established that the OS voltage (or nozzleskimmer bias (NS) in other instruments4*’)has a dramatic effect on the charge state distribution observed in ESI-MS, as well as on the extent of fragmentation and solvent adduction. Consequently, differences between the myoglobin spectra in Figures 2 and 3 and in spectra published elsewhere&reflect different OS voltages. The OS voltages employed here were selected to optimize detection of (M + H)+. Spectra intended for direct comparison, e.g., the upper and lower panels of Figure 3, employ the same OS voltages. Unassigned background ions (labeled B) are attributed to impurities in the methanol. The background ion spectra differ in Figures 2 and 3 because the different OS voltages employed lead to differing amounts of ion-molecule reaction or collisional dissociation. Any conclusions based upon a comparison of attenuation in the myoglobin (M + nH)“+ charge-state distribution arising from reaction with fluorescein and AMP must await further studies for which reactant solutions, reaction temperatures, gas flows, and mass spectrometer tuning effects upon ESI spectra are carefully accounted. The observed (M H)+ cations probably arise from charge inversion of (M - H)- anions, since the (M + H)+ intensity closely follows that of the anion. That is, if operating conditions for the negative ESI source are selected to reduce the intensity of (M - H)- in single ESI source operation, then the (M H)+ intensity is also reduced in dual-source operation. This dependence was explored further for fluorescein by adjusting the pH of the bulk solution to reduce the concentration of (M - H)-relative to the neutral species. We observed that the (M + H)+ intensity (from dual-source operation) was reduced by roughly the same factor as was the singlssource (M - H)- intensity. Although adenosine’s high proton affinity” might suggest the possibility of protonation of the neutral nucleotide as the source of (M H)+, there is no evidence of abundant generation of nonvolatile neutral species by ESI s o u r c e ~ . ~ ~ *The ~ ~experimental ,l* conditions generally employed ensured that the AMP and fluorescein were almost entirely ionized in solution. Proton affinities, for the relevant neutral species would suggest that any neutral species capable of deprotonating myoglobin (M nH)”+should also extract protons from (H20),H+ ions (of low n), contrary to our observations. Thus, it seems unlikely that the positive ions observed arise from AMP or fluorescein neutral species introduced from the ESI source. The apparent lack of charge balance indicated in Figures 2 and 3 requires some comment. One might expect that operation of the second (positive) ESI source would reduce the AMP or fluorescein (M - H)- intensity by an amount equal to the intensity of (M + H)’ or by a larger amount if neutralization (M formation) is significant. The myoglobin charge-state distribution should shift to lower charge, but the integrated intensity over all charge states should remain constant. However, we observe little attenuation of (M - H)- from dual-spray operation. The AMP (M - H)- anion is perhaps 2 orders of magnitude more intense than the (M H)+cation, while spray stability is insufficient to resolve less than a few percent difference in the (M - H)-signal. The fluorescein (M - H)-anion was approximately 20 times as intense in single-spray operation as was the (M + H)+cation in dual-spray operation, while a reduction of at most 10%was observed in the anion signal under dual-spray operation. However, intensity variations of up to 10% were common when toggling the negative ESI source on and off. Future modifications to the source should reduce such instabilities. At present we can estimate only roughly the relevant reaction rates. On the basis of gas flow rates and ion currents through the capillary we can estimate anion concentrations at lo8 ions/cm3 and reaction times at - 5 X lo4 s. On the basis of literature rate constants for fast ion-ion recombination or neutralization reactions (1@-10-8 cm3/s) half-lives of 0.1-1 s might be expected, not inconsistent with the small attenuation observed. However, such rates are inconsistent with the large attenuation
+
(M+H)+
B* Downloaded by NEW YORK UNIV on September 13, 2015 | http://pubs.acs.org Publication Date: August 1, 1991 | doi: 10.1021/j100170a006
Letters
+
+
600
400
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mlz Figure 3. Top: positive ion spectrum obtained from positive ESI of horse heart myoglobin (M’; OS = 500 V). Bottom: positive ion spectrum obtained from positive ESI of horse heart myoglobin and negative ESI of fluorescein (M,M,332). B = background. Both spectra are plotted on the same absolute intensity scale.
Similar studies were conducted using a solution of disodium fluorescein (structure 11) with water in the negative ESI source,
,
O/
/ W
0
+
oco2H /
fluorescein (II)
yielding (M - H)-anions at m / z 331. Although fluorescein can be present as a stable cation, neutral, monanion, or dianion in solution,I6 the gas-phase dianion was not observed under the conditions employed. When ions from positive ESI of myoglobin were introduced through the other inlet, the negative ion spectrum was qualitatively unchanged. The positive ion spectrum included protonated myoglobin species and an ion at m / z 333. Consistent with the latter’s assignment as the fluorescein (M + H)+cation, it was present only with both electrospray ion sources operating (see Figure 3). As observed previously for AMP, the (M + H)+ ion was also detected upon substitution of myoglobin with solutions generating other highly charged ions but not with solutions generating mostly singly charged species. The ion was also not observed with bradykinin, which produces primarily (M + 2H)2+ species, along with less abundant singly and triply charged ions. (See Table 1.)
+
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~~~
(16) (a) Dichl, H.;Horchak-Morris,N. Talanra 1987.34, 739-741. (b) Dichl, H. Talanta 1989, 36, 413-415. (e) Diehl, H.; Markuszewski, R. Talanta 1989, 36, 416418.
(17) Grew, F.; Liguori, A.; Sindona, G.; Uccella, N. J . Am. Chcm. SOC. 1990, I 12.9092-9096.
J. Phys. Chem. 1991, 95,6415-6417
of the most highly charged myoglobin charge states (observed for the AMP studies). While the reduced intensities of the higher charge states may simply reflect a shift in the entire distribution to lower charge state, the high attenuation rat@ lead us to believe that ion-neutral processes may also contribute. Indeed, studies by McLuckey et ai. have shown that ion-molecule reactions can preferentially attenuate higher charge states." It is believed that Coulombic forces result in the higher charge states being more reactive. Uncertainties in the reaction rates and thermodynamics of their reactions presently preclude unambiguous interpretation of our results. We believe that under our conditions, reaction with multiply protonated macroions could be a requirement for detectable charge inversion of singly charged anions (see Table I). A "one-step" multiproton transfer (reaction 1) could have substantially larger (M - H)- + (M' + nH)"+ (M + H)+ + (M' ( n - 2)H)("2)+ (1) rates than a two-step sequential charge inversion (Le., an ion-ion reaction followed by an ion-molecule reaction). Although the first neutralization step is likely also to occur, we believe it is unlikely to contribute to the charge inversion process due to a rate-limiting reionization step. One might also expect the ionDownloaded by NEW YORK UNIV on September 13, 2015 | http://pubs.acs.org Publication Date: August 1, 1991 | doi: 10.1021/j100170a006
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6415
neutral process would have a collision cross section substantially lower than that of the initial ion-ion step. A single-step charge inversion process would explain both the high effciency for overall inversion and the requirement for multiple protonation of the positive ion reactant. The evidence presented for charge inversion in the reaction of singly charged anions with multiply charged macroions raises many questions for future ion-molecule and ion-ion reaction studies. We wonder about the roles of ion structure, energetics, and chemical equilibrium on these reactions. If we assume that the charge inversion occurs as a result of a single ion-ion encounter (reaction l), how rapidly do the two protons transfer and how might that transfer be influenced by the chemical nature and proximity of the charge sites? What role, if any, is played by associated solvent molecules and the bath gas? Further investigations are being initiated to address these issues. Acknowledgment. We thank C. G. Edmonds, J. A. Loo,and J. L. Fulton for helpful discussions and the U.S.Department of Energy for financial support through internal Exploratory Research of the Molecular Science Research Center (Contract DE-AC06-76RLO 1830). Pacific Northwest Laboratory is operated by Battelle Memorial Institute.
Nascent SO(X%) Vibrational Distributions from the Photodissociation of SO2, S0Cl2, and (CH3)2S0at 193 nm Xirong Cben, Federico Asmar, Hongxin Wang, and Brad R. Weiner* Department of Chemistry, University of Puerto Rico. RIo Piedras, Puerto Rico 00931 (Received: May 29, 1991; In Final Form: June 28, 1991)
Nascent vibrational skate distributions of SO(X3Z-) have been measured by laser-induced fluorescence spectroscopy following the 193-nm photodissociations of SO,,SOCI,, and (CH3),S0 in the gas phase. All three of the distributions are found to be inverted, indicating strong dynamical effects in the photodissociation processes. A Franck-Condon/golden rule model is used to propose mechanisms to account for the experimental vibrational distributions.
Introduction The detailed mechanisms of photodissociation processes can be revealed by measurement of the nascent vibrational state distributions of one or more of the photofragments following photoactivation of a polyatomic molecule.' Information about the nature of the excited species immediately prior to the fragmentations can be obtained from this data. In the case of multiple bond dissociations in polyatomics, nascent vibrational state distributions can help elucidate whether photofragmentationoccurs stepwise or in concert.' We report in this Letter the measurement of nascent vibrational state distributions of the SO(X3Z-) fragment obtained in the 193-nm photodissociationsof sulfur dioxide (SO,), thionyl chloride (SOCl,), and dimethyl sulfoxide ((CH,),SO). The nascent vibrational distributions were measured by laser-induced fluorescence (LIF) spectroscopy of SO(A311-X3Z) transition in the wavelength region of 255-295 nm. Experimental Section The experiment employs a typical two-laser, pump-probe apparatus, which has been described previously.2 Briefly, low pressures (0.01405 Torr) of SO,, SOCl2,or (CH3)$0 are flowed through a reaction chamber comprised of a four-way stainless steel cross fitted with scattered-light-reducing, brass extension arms To whom correspondence should be addressed.
0022-365419112095.64 15$02.50/0
on two opposite sides. Gas inlets are located on the extension arms, and the gas outlet is on the four-way cross. The reaction cell is pumped by a 2-in. diffusion pump, and the entire chamber and vacuum system can be evacuated to lo-' Torr, routinely. Cell pressures are measured at the exit of the cell by a capacitance manometer. Reactant gases are photolyzed with the 193-nm output (10-50 mJ/cm2) of an excimer laser (Lambda Physik LPX205i) operating on the ArF transition. Nascent SO photofragments are monitored by LIF on the (A-X) transition in the 255-295-nm region of the spectrum. The probe laser light in this region was generated by frequency doubling (8-BaB204crystal) the output of a Lambda Physik FL3002 tunable dye laser (dye is Coumarin 503 or Coumarin 540A), which is pumped by a Lambda Physik LPX205i excimer laser at 308 nm. The lasers are collinearly counterpropagated along the extension arm axis to maximize the overlap region in the center of the reaction chamber. Fluorescence is viewed at 90° relative to the laser beam axis by a high gain photomultiplier tube (Hamamatsu R943-02) through a long-pass filter (Schott WG295). The output of the photomultiplier tube is processed and averaged by a gated inte( 1 ) Bersohn, R. In Molecular PhorodissociaiionDynamics; Ashfold, M.
N.R., Baggott, J. E., Eds.; Royal Society of Chemistry: London, 1987; pp 1-30. (2) Barnhard, K. 1.; Santiago, A.; He, M.; Asmar, F.;Weiner, E. R. Chem. Phys. Lett. 1991, 178, 150.
0 1991 American Chemical Society
