J . Phys. Chem. 1986, 90, 3994-3998
3994
State Wstrtbutlons, Quenchtng, and Reaction of the PO Radlcal Generated In Exclmer Laser Photofragmentatton of Dimethyl Methylphosphonate Rosario C. Sausa,t Andrzej W. Miziolek, Ballistic Research Laboratory, Aberdeen Proving Ground, Maryland 21 005
and S . Randolph Long* Chemical Research and Development Center, Aberdeen Proving Ground, Maryland 21 01 0 (Received: December 3, 1985; In Final Form: February 21, 1986)
Focused KrF and ArF excimer laser radiation acting on dimethyl methylphosphonate (DMMP) produces ground electronic state PO radicals via an initial two-photon absorption by the DMMP parent followed by a sequence of daughter photofragmentations. Probing the PO radical by laser-induced fluorescence,utilizing the A2Z+-XzII transition near 247 nm, reveals that the nascent PO rotational population has a distribution characterized by a temperature considerably greater than 300 K, while at least 95% of the PO radicals are formed in the lowest vibrational state. The nascent spin-orbit distribution in the zI11,2,,,2state is near that characteristic of 300 K. Quenching of the A state of PO by both nitrogen and oxygen occurs with a rate constant of about 1.8(*0.5) X cm3 molecule-I s-'. Ground-state PO reacts bimolecularly with molecular oxygen with a rate constant of 1.2(f0.2) X IO-" cm3 molecule-I s-', equivalent to about 1/40 the hard-sphere collision rate.
Introduction Many molecules are difficult to detect and identify spectroscopially due to the absence of well-defined rotational structure in their infrared absorption bands and to the lack of structured or any absorption in the convenient ultraviolet-visible spectral region. As a means of detection and class identification of such molecules, we are pursuing an approach based on the observation of numerous studies over the past few years that focused excimer laser radiation can lead to substantial fragmentation of a parent compound. (A partial list of representative studies is provided in ref 1.) The photofragments typically include di- and triatomics (as well as atoms), which do generally have structured, readily identifiable transition systems in the ultraviolet-visible spectral region. These fragments may be detected by their prompt emission, if formed in electronically excited states, or by probe techniques such as laser-induced fluorescence (LIF) or laser multiphoton ionization (MPI). Detection of distinctive photofragments can lead to identification of the parent. In particular, strongly bound moieties characteristic of a class of compounds may be anticipated to survive the focused laser photolysis. The dimethyl methylphosphonate molecule, (CH30)2P(=O)CH3, hereafter referred to as DMMP, is one which has the spectroscopic difficulties mentioned earlier. Its infrared absorption bands are characterized by a very close, unresolvable rotational structure, while the molecule does not absorb between the near-infrared and 200 nm in the ultraviolet. The PO double bond, common to all organophosphonates, has a strength of about 135 kcal/mol: significantly stronger than the other bonds involving the phosphorus atom ( P a , 86 kcal/mol; P-C, 65 kcal/mol; 042, 85 kcal/mol).* The COzlaser IR multiphoton dissociation of DMMP and other organophosphorus molecules in a molecular beam has been studied by Chou, Sumida, and Wittig.3 Two-color MPI via the BZZ+ state was used to examine the nascent population distributions in the PO fragment. In a prior publication on the focused excimer laser fragmentation of DMMP,' we reported on the LIF detection of the PO radical via its A22+-X211, transition4 with origin near 247 nm. In that work, formation of the PO radical was established by A-X spectroscopy, the excimer laser power dependence of PO production was determined to provide some insight into the dissociation mechanism, and the A state lifetime was measured to NAS/NRC Postdoctoral Research Associate. Current address: IBM Watson Research Center, P.O.Box 218, Room 6-08, Yorktown Heights, N Y 10598.
be 9 f 2 ns, in good agreement with a measurement by Anderson and co-w~rkers.~ This report provides an account of our further studies on the excimer laser dissociation of DMMP with regard to identification of other fragments generated as well as the evolution of PO population distributions with collisions, quenching of the A state by nitrogen and oxygen, and reaction of PO with molecular oxygen. The latter two aspects of this work are of particular relevance to the use of the photodissociation/LIF scheme for detection of organophosphonates under ambient conditions, Le., in the atmosphere.
Experimental Section The techniques used in this study have been described in detail previously.' Briefly, a Lumonics Model 861 excimer laser operating as an argon fluoride (193 nm) laser or a krypton fluoride (248 nm) laser was employed as the photolysis source. Pulse energies of up to 25 mJ from ArF and 60 mJ from KrF were available in the photolysis cell, a six-way cross fitted with Suprasil windows on optical ports. The excimer laser pulse duration is 10-15 ns. A nominally 200-mm focal length Suprasil lens focused the photolysis beam into the cell. The photolysis laser energy was varied by inserting dielectric filters (Acton Research) into the laser beam and was measured by a volume absorbing calorimeter (Scientech). Fluorescence of the PO radical was induced by radiation with wavelengths near 246 nm generated in a Quanta-Ray Nd:YAG-pumped dye laser system. The pulse energy of this probe laser beam was typically about 0.35 mJ with a pulse duration of about 6 ns fwhm. The probe beam was not focused and counterpropagated through the photolysis cell along the same axis as the photolysis laser. A master trigger system allowed the (1) Long, S. R.; Sausa, R. C.; Miziolek, A. W. Chem. Phys. Lett. 1985, 117, 505. (2) Corbridge, D. E. C. Phosphorus: An Outline of Its Chemistry, Biochemistry, and Technology; Elsevier: New York, 1978. (3) Chou, J. S.; Sumida, D. S.;Wittig, C. Chem. Phys. Letr. 1983, 100, 397. J. Chem. Phys. 1985, 82, 1376. (4) The following papers provide detailed spectroscopic data on the A-X transition generated in arcs: (a) Dixit, M. N.; Narasimham, N. A. Proc. Indian Acad. Sci. 1968, A68, 1. (b) Coquart. B.; Couet, C.; Tuan Arh, N.; Guenbaut, H. J. Chim. Phys. 1967,64, 1197. (c) Rao, K. S. Can. J . Phys. 1958, 36, 1526. (5) Anderson, W. R.;Decker, L.J.; Kotlar, A. J.; DeWilde, M.A. Paper RC8, Proceedings of Thirty-Ninth Molecular Spectroscopy Ohio .. Symposium, - . State University, 1984.
This article not subject to US.Copyright. Published 1986 by the American Chemical Society
Laser Photofragmentation of DMMP
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3995
I 6.0 -
1 €
5.01
E
4.0-
Y
1
~ " "260~ " "270" ' " 28'C
250
2*ot i'
-WAVELENGTH ( nm )Figure 1. LIF spectrum of the PO radical in the A-X transition excited spin-orbit component of at the (0,O)P2 bandhead (involving the zI13/z the ground state). The probe laser follows by 350 ns the action of a focused KrF (248 nm) laser on -0.5 Torr of DMMP.
delay between the photolysis and probe lasers to be varied from 0 to 2 ps for these studies. Fluorescence was viewed perpendicularly to the laser optical axis and collected by 75-mm-diameter quartz lenses (100- and 225-mm focal lengths) onto the slit of a 0.35-m McPherson monochromator fitted with a 1200 g/mm grating blazed for either 250 or 500 nm. Generally, an EM1 9558QA photomultiplier was employed and coupled to a PAR 162 boxcar averager for spectral recording or to a Tektronix 7912 AD digitizer (7A24 amplifier with 0.9-ns rise time, 7B90P time base) for amplitude measurements. For lifetime measurements, a faster (1.5 ns) rise time RCA 4832 PMT was used in conjunction with the waveform digitizer to provide a net 1.8-11s rise time system. Digitizer output was accumulated in a PDP 11/04 computer. Dimethyl methylphosphonate (Alfa Products) underwent multiple freeze-thaw cycles before each set of experiments. Nitrogen (99.9%) and oxygen (99.999%), both Matheson, were used without further purification. Experiments were conducted under flow conditions. For studies using nitrogen or oxygen, these gases were introduced by flowing them directly over the liquid DMMP sample (vapor pressure of 1 Torr at 25 "C). Cell pressures were measured with a Datametric 1012 Barocel capacitance manometer.
Results and Discussion Identification of Excimer Laser-GeneratedPhotofragments and Dissociation Mechanism. Under focused ArF laser photolysis, emissions due to electronically excited CH near 314 nm (C2Z+ X211) and near 432 nm (A*A X211) and to excited C atom ('PI 'So) a t 248 nm are observed. These emissions could not be detected when the ArF laser was not focused nor when the KrF laser, focused or unfocused, was used for photolysis. In fact, we observe no photochemistry when the excimer lasers are applied without focusing. The appearance of C atom emission is the result of a fortuitous overlap of the 193-nm ArF radiation with the 'PI-'D2 transition of the C atom at 193.1 nm.6 The ArF laser thus acts as a probe for C atoms in the metastable 'D2 state lying 10 193 cm-' above the ground 3P2state. The fluorescence a t 247.9 nm arises from the 'PI 'So transition. As noted in our previous publication,' we detect the PO radical as a photoproduct formed in the ground X211 state using laser-
--
-
-
(6) Moore, C. E. Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. 1971,No.
35.
O'
0!5
-h
lb
115
20
215
-
310 315
( LASER ENERGY ) Figure 2. Log-log plots of emission vs. excimer energy: (A) CH(2A) emission from focused ArF radiation on D M M P (B) PO(A-X) LIF from focused ArF radiation on DMMP; (C) PO(A-X) LIF from focused KrF radiation on DMMP. induced fluorescence via the A22+-X211transition with origin near 247 nm. The v' = 0 fluorescence spectrum is reproduced in Figure 1. The PO radical is produced by both focused ArF and KrF laser photolysis of DMMP. N o excited state PO is generated with either photolysis laser. It is reasonable to suspect that other radicals might be formed by these photolyses, notably methoxy (CH30) and methyl (Cfi,) radicals. We sought to detect C H 3 0 by LIF via its A2Al-X2E transition near 310 nm7 after having gained experience with this radical by detecting it via LIF in the unfocused ArF laser photolysis of methyl nitrite (CH,ONO). We could readily detect methoxy radical in the latter experiment but found none in the case of focused excimer laser dissociation of DMMP. We must presume that C H 3 0 has at most a transitory existence during the focused excimer laser photolysis of DMMP and does not survive. The CH, radical is not amenable to LIF detection. It should be possible to detect it by multiphoton ionization, but we have not yet attempted this. There is a good possibility that CH3 also does not survive the excimer laser photolysis. Baronavski and McDonalds observed CH in its A2A and B22- states following focused ArF photolysis of CH3Br. W e have likewise observed the same product in the focused ArF photolysis of CH31. The initial photochemical event for both these halomethanes is most probably elimination of halogen atom. The CH radical must appear therefore as a result of CH, decomposition. We must suspect that CH, radicals formed by photolysis of DMMP have a similar photochemical fate. We have developed log-log plots of C H A2A emission intensity vs. ArF laser energy and for LIF of PO vs. laser energy for both ArF and KrF photolysis. These plots, shown in Figure 2, are characterized by slopes of n = 1.8 for CH, n = 1.9 for PO from ArF photolysis, and n = 2.2 for PO from KrF photolysis. Considering that DMMP does not absorb at either 248 or 193 nm, it is evident that the near quadratic power dependence of C H and (7) (a) Inoue, G.;Akimoto, H.;Okuda, M. J . Chem. Phys. 1980,72,1769. (b) Ebata, T.;Yanagishita, H.; Obi, K.; Tanaka, I. Chem. Phys. 1982,69, 27. (c) Sanders, N.; Butler, J. E.; Pasternak, L. R.; McDonald, J. R. Chem. Phys. 1980, 48, 203. (8) Baronavski, A. P.; McDonald, J. R. Chem. Phys. Lett. 1978,515,369.
3996 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
..
t
.
0
Q,
0
0
0
Sausa et al.
)I
0
0 0
0
0
o
0
J=7
no buffer
IO torr N, I O
0
200
400
600
!
000
DELAY, nsec Figure 3. Time dependence of LIF of PO excited at (0,O) Q,bandhead of the A-X transition, using focused KrF laser (40 mJ/pulse) and 0.5 Torr of DMMP.
PO must be associated with an initial two-photon absorption to induce the DMMP molecule to undergo photochemistry. At laser intensities sufficient to induce this initial transition, all subsequent photolysis events occur via transitions which are saturated. Although the energy deposited in the DMMP by two 248- or 193-nm photons is sufficient to cause dissociation of all three single bonds involving the phosphorus atom, we feel it more reasonable to presume that the fragmentation to bare PO occurs through simple photochemical events involving sequential absorption of several photons. Unfortunately, our data do not permit identification of the immediate precursor of PO in the excimer laser photolysis of DMMP. The overall decomposition process must be quite complicated and several fragmentation routes leading to PO can be envisioned. Should each step in the fragmentation process occur as a simple bond cleavage with the weakest bonds lost early in the sequence, the PO2and C H 3 0 P 0molecules emerge as prominent candidates as immediate precursor to PO. Both these molecules would likely produce PO upon ultraviolet excitation. However, we cannot distinguish between these possibilities nor others that involve more complex photochemical behavior and must leave open a t present the question of precursor identification. Rotational, Vibrational, and Spin-Orbit Distributions of PO. When excited at the (u',~'') 0,OQl bandhead at 246.3 nm, the laser-induced fluorescence of PO monitored at the (0,l) band at 355.4 nm exhibits a rise with increase of probe laser delay following photolysis. Figure 3 depicts this effect with KrF laser energy at 40 mJ/pulse and DMMP pressure at 0.5 Torr. The increase of PO LIF signal with probe delay is exponential, a least-squares fit of the Figure 3 data for DMMP alone yielding a rise time of ca. 220 ns. Also shown in Figure 3 is the effect of adding 10 Torr of nitrogen. In the latter case, the rise time is shortened to ca. 30 ns. There are two causes to which such a rise in the LIF signal may be attributed: (1) the production of PO is due to a reaction, either bimolecular or unimolecular, or (2) collision-induced changes in the PO nascent population occur. Figure 4 demonstrates that it is the latter mechanism which is responsible for the rise of the fluorescence excited at the (0,O)bandhead. This figure compares the excitation spectrum in the (0,O)Q1and P1 bandhead regions for a 50-11s delay and 200 rnTorr of DMMP with that for an 800-ns delay and 400 mTorr of DMMP. It can be seen that the intensity of the Pl bandhead, formed by rotational levels J near 23, decreases and that of the Q1bandhead, formed by rotational levels J near 7, increases as the number of collisions, manifested by increase in pressure and delay, increases. The observed fluorescence signal rise is thus attributable to rotational relaxation of the nascent population, which is considerably warmer than 300 K. The observed rise time of the data for 500 mTorr of DMMP of 220 ns can be cast as a rate of about 10 ps-' Torr-', while the hard-shere collision rate of PO on DMMP may be estimated as 25 flCs-' Torr-'. Roughly three collisions are required for rotational equilibration to a 300 K distribution to occur. A similar result is found when rotational levels near 58 in u" = 0 are examined,
e
i
n
J =23
240.33
-WAVELENGTH
246:47
( nm 1-
Figure 4. Excitation spectrum in region of (0,O) bandhead of A-X(zII,,z) transition. Shaded spectrum: 50-11sdelay, 0.2 Torr of DMMP. Upper spectrum: 800-ns delay, 0.4 Torr of DMMP.
except that here one observes an approximately 200- ns decay as the high rotational levels relax. Unfortunately, due to the dense rotational structure and blending of lines from different rotational branches in the (0,O) band, we are unable to resolve individual lines with our approximately 1-2-cm-' bandwidth probe laser. The most accurate statement we can make regarding the nascent rotational state distribution of PO is that it is much more heavily weighted toward high rotational levels than a 300 K Boltzmann distribution. We find that the distributions generated by the ArF and KrF lasers are qualitatively very similar. In contrast to the high rotational excitation, the nascent vibrational excitation in the PO photoproduct is quite low. In fact, we were unable to detect any population in u" = 1 or v" = 2 of X211 by LIF, using (1,l) and (2,2) bandheads for excitation and monitoring (1,2) and (2,3) bandheads, respectively, for fluorescence. Taking into account Franck-Condon factors for the relevant (O,O), (O,l), (l,l), and (1,2) bands,l0 we should have been able to detect readily a u = 1 population 10% that of u = 0 (the actual signal difference using corresponding Q1 bandheads for excitation would be about 5%). Considering this observation with the 1233-cm-' vibrational spacing of PO X211, the upper limit that can be placed on the effective vibrational temperature is then about 750 K. Since the ground *II state of PO is spin-orbit split into two components, one with Q = 3/2 lying 224 cm-' above the Q = lI2 state, it is of interest to determine if there is any specificity for production of PO in one of these states. To do this, we have examined the relative intensity of LIF excited at the Q1bandhead and that excited (formed by rotational levels J about 7 in Q = at the P2 bandhead (formed by rotational levels J about 7 in 0 = 3/2) as a function of delay at a pressure of 400 mTorr. We find that this ratio remains unchanged, within experimental error (ca. 15%), from 204s delay to 2-ps delay. Certainly thermalization to near 300 K has occurred a t the longest delay (ca. 20 collisions following photolysis) while at 204s delay, only about 20% of the nascent PO radicals have suffered a collision and the (9)Long, S . R.;Christesen, S . D.; Force, A. P.; Bernstein, J. S . J. Chem. Phys. 1986.84,5965. (IO) Suchard, S. N. Specrroscopic Daza, Vol. I, 'Heteronuclear Diatomic Molecules"; IFI/Plenum: New York, 1975.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3997
Laser Photofragmentation of DMMP
c
1
8
v)
3-
1
G;: m
2-
e Y
1-
I
,
0
,
2.5
0
r
I
5
7.5
1'0
NITROGEN PRESSURE, TORR Figure 5. Stern-Volmer plot of observed A22+ PO fluorescence decay rates vs. nitrogen pressure. The slope yields a rate constant of 1.8 X cm3molecule-' s-' (5.7 ps-' Torr-') for quenching of the A state by N2.
0
2.5
7.5
5
0
10
OXYGEN PRESSURE, TORR Figure 7. Plot of observed first-order rate constants for PO decay vs. pressure of O2 The bimolecular rate constant, 1.2 X lo-" cm3molecule-' s-I (0.32 ps-l Torr-'), is obtained from the slope. of added oxygen. In all these studies, the focused KrF laser was used for photolysis and the DMMP partial pressure was roughly 200 mTorr. The loss of PO in the presence of 500 mTorr of DMMP alone is seen to be quite small on the 2-ps time scale and may be due to diffusion out of the observation region and/or slow reaction with parent or other radicals. The addition of oxygen to the flow results in an exponential loss of PO a t all pressures studied (2-10 Torr). The observed first-order rate constants over the O2pressure range studied are accumulated in Figure 7,which demonstrates that the observed rates are linearly dependent on oxygen pressure. These data, taken together, suggest that the loss of PO is due to a simple bimolecular reaction between PO and 02.We presume this reaction to be PO 0 2 --PO2 0 (1)
+
DELAY, pSEC Figure 6. Semilog plot of decay of PO XzII v"= 0 (monitored by LIF) in presence of 5 Torr of O2 (circles). Upper plot shows variation of PO concentration with 0.5 Torr of DMMP alone (crosses).
measured intensity ratio would still reflect a pronounced specificity in the nascent distribution. We conclude that there is no particular specificity in the PO spin-orbit state distribution in the excimer laser dissociation of DMMP. Both spin-orbit states therefore appear to correlate with the immediate precursor of PO. This finding, which applies to both ArF and KrF photolyses, is in significant contrast to the results of Chou, Sumida, and Wittig,' who found in the C02 laser IRMPD of DMMP and similar molecules a substantial propensity for production of the = 1/2 component in nascent PO. These observations are not, however, in contradiction, since the photolysis techniques in the two studies are different (focused IR vs. focused UV) and the immediate precursors may be different (Chou et al. suggest CH30P0as the PO precursor in their work). Rate of Quenching of AZZ+ and Reaction of %Il with Oxygen. In a previous publication, we reported the measurement of the PO A2Z+ radiative lifetime to be 9 f 2 ns.' We have since developed Stern-Volmer plots of the inverse lifetime of the A state vs. pressure in the range 1 to 10 Torr for nitrogen and oxygen as collision partners. Such a plot for nitrogen is shown in Figure 5 . In all our measurements, the 6-11s probe laser pulse width is deconvoluted from observed waveforms. The slope of the Stern-Volmer plot, which arises from the relation kobd = kmd kQ[M], yields a quenching rate constant kQ = 1.8(f 0.5) x cm3 molecule-' s-' ( - 5 ps-' Torr-') for nitrogen. (The purely statistical error evaluation is at the 95% confidence level.) This rate is near collisional, as the hard-sphere collision rate for PO on nitrogen is about 12 ps-l Torr-'. With oxygen used as the collision gas, we find that quenching of the A state occurs with the same rate as for nitrogen. Reactive decay of PO occurs in the presence of oxygen. Figure 6 is a semilogarithmic plot of the PO u = 0 LIF intensity, excited at (0,O)QI bandhead, vs. probe laser delay in the case of 5 Torr
+
+
with a AH zi 1.6 f 4 kcal/mol." The bimolecular rate constant we obtain is 1.2 (f0.2) X lo-" cm3 molecule-' s-' (-0.32 ps-' torr-'). The stated error corresponds to the 95% confidence level and is purely statistical. Estimating the hard-sphere collision rate for PO on O2 as 12 ps-' Torr-' shows that about one in forty collisions results in a reactive event. Two other studies of this reaction, one previousI2 and one concurrent" with both using flow discharge systems, have produced rate constants attributed to the above reaction which are 50 times slower than we report. There are two possible mechanisms to be considered which might lead to an erroneous result in our measurement. First, a photoproduct other than PO may react very rapidly (near collisional rate) with 02, with the product of that reaction reacting with PO on the time scale we observe. This possibility may be discounted by noting that, in this case, the PO loss rate would be independent of the O2pressure (since it is in excess) and would be dependent on the photolysis laser energy (since the photoproduct concentrations are probably at least quadratically dependent on laser power). We find the converse to be true: the loss rate is linearly dependent on O2 concentration and a factor of two change in excimer laser power has no effect on the measured loss rate. The second mechanism to be considered is that O2may be dissociated by the focused KrF laser and we observe the reaction of PO with 0 atoms. We do observe significant deviation from exponential decay when the photolysis source is the shorter wavelength ArF laser, which is well-known to generate 0 atoms by single photon-induced photochemistry of O2in the Schumann-Runge continuum. In order (11) As suggested by ref 13 using Benson's tables (Benson, S. W. Thermochemical Kinetics; Wiley: New York, 1968) and AHfZg8= -60.5 2.9 kcal/mol for PO2 from Guido et 81. (Guido, M.;Balducci,'G.; DeMaria, G.; Gigli, G. J. Chem. Soc., Faraday Trans. I , 1977, 73, 121). (12) Aleksandrov, E. N.; Arutyunov, V. S.;Dubrovnia, I. V.;Kozlov, S. N. Dokl. Akad. Nauk SSSR 1982, 267, 110. (13) Wong, K. N.; Anderson, W. R.;Kotlar, A. J.; DeWilde, M. A. J. Chem. Phys. 1986,84, 81.
*
J. Phys. Chem. 1986,90, 3998-4001
3998
to discount the 0 atom mechanism most straightforwardly, we draw upon a very recent measurement by Long et al.9 of the PO loss rate in the C 0 2 laser multiphoton dissociation of DMMP in the presence of 02.This photolysis technique cannot generate 0 atoms by photochemistry in 02.The measured bimolecular rate constant in that study is 1.4(f 0.2) X lo-" cm3 molecule-' s-', in good agreement with the rate constant obtained with the KrF laser. There is accordingly little doubt that the rate of loss of PO in the presence of O2 measured via the laser photolysis technique is in fact attributable to the bimolecular reaction of PO with 02. Several possible causes of the discrepancy of this rate constant measured by laser photolysis vs. flow discharge system may be cited. As noted by Wong et al.," reactive loss of PO may occur by a mechanism more complex than the simple reaction 1, and the measured rate may be a function of pressure, time scale of measurement, etc. Whereas the laser photolysis studies are conducted on microsecond time scales with 0 2 (and total) pressura from 0.2 to 10 torr, the flow discharge system measurements of Wong et al. involve millisecond time scales and O2pressures below 0.1 torr (though buffered to 2 torr total with argon). Alternatively, the comparatively rapid rate we measure may indicate a relatively low reaction barrier for reaction 1. Coupled with the near thermoneutrality of reaction 1, this could suggest a facile reverse reaction. One would then need to be concerned about the possible establishment of equilibrium between reaction 1 and its reverse in an experiment which incorporates a long contact time between reactants, as in the flow discharge experiments. However, the temperature dependence of reaction 1, as well as accurate knowledge of the initial concentrations of 0 atoms and PO2 in the flow discharge experiments, would be required to assess whether establishment of equilibrium is of legitimate concern. At present, the cause of the discrepancy in rate constant measurements is still uncertain.
photons in sequence to form the ground electronic state PO radical, among others. This photochemical behavior complies with the expected tendency of strongly bound moieties to survive focused UV laser fragmentation of parent molecules containing such moieties. Laser-induced fluorescence via the A2Z+-X211transition indicates that the PO is formed rotationally warm and vibrationally cool. We have found no evidence of any preference for formation of PO in the i2 = 1/2 or i2 = 3/2 component of the 211 ground state. In fact, the nascent spin-orbit distribution is not far from a 300 K distribution. Several rates relevant to the viability of the laser photofragmentation/LIF scheme for detection of phosphonates under ambient conditions have been measured. The PO A state radiative lifetime of 9 ns demonstrates that the A-X transition is very strong and suggests that it is a particularly sensitive spectroscopic vehicle for PO detection. Rates of quenching of the A state by nitrogen and oxygen, coupled with the A state radiative lifetime, indicate that a fluorescence quantum yield of 3% may be expected at atmospheric pressure. About one in forty collisions of PO with oxygen results in reactive loss of PO (presumably to form PO2). This relatively rapid reaction rate suggests that the probing laser should follow the photolysis laser closely in time. For the case of DMMP, where only u = 0 PO is produced by the focused excimer laser photolysis, this restriction is of little consquence since rapid rotational equilibration ensures near complete thermalization of nascent PO within a few nanoseconds under ambient conditions. For this example, then, a single KrF excimer laser tuned14for 0-0 A-X LIF probing of the PO product could serve very well the dual role of photolysis and probe.
Conclusion Following a simultaneous two-photon absorption of focused 193or 248-nm laser radiation, the dimethyl methylphosphonate molecule undergoes photochemistry and its daughter radicals subsequently fragment through absorption of additional ultraviolet
Registry NO. (CH,O),P(--V)CH,, 756-79-6; PO, 14452-66-5; CH, 3315-37-5; C, 7440-44-0; C2, 12070-15-4;0 2 , 7782-44-7; N2, 7727-37-9.
CN(A-X,B-X) Energy
Acknowledgment. A. W.Miziolek acknowledges support for this work from the Chemical Research and Development Center. R. C. Sausa acknowledges support from the National Reserch Council. We thank W. R. Anderson for making available a preprint of his work.
(14) Loree, T. R.; Butterfield, K. B.; Barker, D. L. Appl. Phys. Lett. 1978, 32, 171.
Chemiluminescence in the Reaction of C+(2P) with N20 at Thermal
Masaharu Tsuji,* Ichiro Nagano, Toshihiko Susuki,+Kazumi Mizukami,t Hiroshi Obase, and Yukio Nishimura Research Institute of Industrial Science and Department of Molecular Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 81 6, Japan (Received: December 9, 1985; In Final Form: March 5, 1986)
-
The thermal-energy ion-molecule reaction C+(2P)+ N 2 0 CN* + NO' has been studied by observing CN(A-X,B-X) chemiluminescence in a flowing afterglow. The CN(A-X) emission from u' = 2-1 3 was identified in the 470-800-nm region, while the CN(B-X) emission produced exclusively from rotational perturbation between CN(A:u' = 10) and CN(B:u' =0) was observed in the 386-422-nm region. The branching ratios for the two chemiluminescent reactions, CN(A-X,B-X) and CO+(A-X), in the C+ + N 2 0 reaction were estimated to be 0.62 f 0.04 and 0.38 f 0.03, respectively. The vibrational distribution of CN(A) was found to peak at v' = 5 and the average fraction of vibrational energy deposited into CN(A) was estimated to be 0.31 f 0.03. The specific formation of CN(A) could be explained in terms of adiabatic correlation diagram.
Introduction The observation and analysis of chemiluminescence from ionmolecule reactions provide detailed infarmation on internal-en-
co.,Saiwai-ku, Kawasaki 210, Japan. 'Present address: Nippon Steel Co., Nakahara-ku, Kawasaki 21 1, Japan.
t Present address: Toshiba
0022-3654/86/2090-3998$01.50/0
ergy-state distributions of products and reaction dynamics. In recent years there have been several experimental studies on chemiluminescent ion-molecule reactions between the ground C+(2P) state and small molecules because of their importance in atmospheric and astrophysical processes. Ottinger and his coand Harris et have investigated the following 0 1986 American Chemical Society
