J. Phys. Chem. 1982, 86, 4467-4469
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Photochemistry of Benzene and Oxygen in Supersonic Cluster Beams J. L. Knee, C. E. Otls, and P. M. Johnson' Department of Chemisfry, Sfate Unlversify of New York, Stony Brook, New York 11794 (Received:April 13, 1982; I n Final Form: September 11, 1982)
A reaction is seen to occur between benzene and oxygen in a supersonic beam when mixed clusters are irradiated by 193 nm. This reaction has been investigated by multiphoton ionization mass spectrometry, with primary reaction products including the introduction of up to four oxygen atoms onto the benzene, as well as species resulting from hydrogen abstraction. A two laser experiment indicates that the initial step in the photochemical reaction results in a neutral species which lives for longer than 100 ps. Introduction While of great intrinsic interest, some of the primary processes of condensed-phase chemistry are very difficult to study because of interfering secondary reactions. For example, radical formation often leads to polymerization, obscuring the investigation of dissociation channels. If the condensed-phase system were limited in size and isolated from interfering interactions, primary chemical processes could be investigated with a minimum of complications. Clusters produced in supersonic expansions hold the promise of creating small solid-state-like systems in which chemistry can be observed and analyzed by using powerful techniques, such as high-resolution spectroscopy and mass spectrometry, which have previously been tools used only in gas-phase noncondensed investigations. The benzene-oxygen system comprises a good object of study because of its relevance for combustion processes, because of the extensive spectroscopy which has been carried out on benzene in supersonic expansions,1*2and because of information from recent crossed-beam reaction s t ~ d i e s .Here ~ ~ ~we will describe the 193-nm photochemical reactions of benzene-02 clusters as detected by multiphoton ionization mass spectrometry. Experimental Section Benzene-oxygen clusters were formed by expanding the two gases through a pulsed solenoid valves in a helium carrier gas. The benzene was at its room temperature vapor pressure of around 80 torr while the oxygen pressure was about 10% of the total presaure of greater than 40 atm. Benzene clusters up to about 30 members were seen in this type of expansion, although there may have been larger ones which were missed because of mass-spectrometer tuning. We were not successful in seeing reactions in pure oxygen-benzene expansions, presumably because insufficiently low temperatures were obtained to incorporate oxygen into the clusters. The expansion is skimmed 4 cm downstream by a 1.3mm skimmer6 and the resulting cluster beam irradiated by the laser beam a total of 10 cm from the nozzle. Ions created by the irradiation were repelled into a time-offlight mass spectrometer with a 75-cm flight path. The detector is a Johnston Inc. focussed mesh ion multiplier. Time-resolved signals from each laser pulse are amplified by a Comlinear Model CLClOO preamplifier and trans~~
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(1) J. B. Hopkins, D. E. Powers, and R. E. Smalley, J. Phys. Chem., 85, 3739 (1981). (2)P. R.R. Langridge-Smith, D. V. Brumbaugh, C. A. Haynam, and D.H.Levy, J. Phys. Chem., 85, 3742 (1981). (3)T. M. Sloane, J. Chem. Phys., 67, 2267 (1977). (4)S.J. Sibener, R. J. Buss, P. Casavecchia, T. Hirooka, and Y.T. Lee, J. Chem. Phys., 72,4341 (1980). (5)C. E. Otis and P. M. Johnson,Rev. Sci. Instrum., 51, 1128 (1980). (6) W.R.Gentry and C. F. Giese, Rev. Sci. Instrum. 46, 104 (1975). 0022-385418212086-4467$01.25/0
ferred to a LeCroy Model WD8256 transient digitizer which passes the data to a LSI-11 computer for signal averaging, data processing, and plotting.
Results Various wavelengths of light were used to create the MPIMS spectra. Irradiation by a doubled dye laser in the 6; lBzutransition and 249-nm light from a KrF laser produced spectra identical with each other whether or not oxygen is included in the expansion. Although large clusters of benzene are seen, no masses corresponding to oxygen molecules or oxygen atoms incorporated into these clusters are recorded by using 249 nm except weakly at the very highest laser powers. When the mixed clusters are irradiated with weakly focussed 193 nm, however, oxygen atoms and molecules are retained in the clusters and the correspondinq masses are seen strongly in the spectra. Without inclusion of oxygen, the spectra are the same as if using the lower energy photons. The higher energy light is resonant with the lBlu state of benzene. In spite of being only vibronically allowed this state has an oscillator strength of about 0.1' because of its proximity to the allowed lElu state. 193 nm is near the maximum of the absorption of the lBlu state, while it is resonant only with the low-lying Schumann-Runge vibrations of oxygen, which have low Franck-Condon factors The abin a total oscillator strength of only 3 X sorption of oxygen is therefore expected to be more than three orders of magnitude smaller than benzene at the ArF wavelength. As a result any reaction in a cluster would be more probable to be initiated by benzene absorption than by direct O2 predissociation. Absorption of oxygen not contained in clusters is also practically eliminated because the laser travels several meters through air before intersecting the molecular beam. Oxygen absorption bands are therefore removed from the laser profile, making recoil collision of atoms from free oxygen molecules improbable. The mass spectra of perdeuteriobenzene under 193-nm excitation, with and without oxygen included, are shown in Figure 1. It is seen that, in addition to the normal fragmentation spectrum and the pattern of benzene clusters seen in the spectrum of benzene without oxygen, there are peaks at masses corresponding to inclusion of up to four oxygens (but no more) on a single benzene parent, as well as new fragment masses. Major observed masses are listed in Table I, along with the probable atomic formula which each mass represents. Ambiguities in the (7)J. R.Platt and H. B. Klevens, Chem. Rev., 41, 301 (1947). (8)R.H. Huebner, R. J. Celotta, S. R. Mielczarek, and C. E. Kuyatt, J. Chem. Phys., 63, 241 (1975).
0 1982 American Chemical Society
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TABLE I: New Species Which Appear in the Mass Spectra from Fragmentation of Clusters When Oxygen is Added to the Benzene-He/Perdeuteriobenzene-He Mixturesa C6H6
C6D6
mass
30'
.1_*-+_1
50
100 M A S S , AMU
150
200
Flgure 1. Comparison of perdeuteriobenzene cluster mass specma (a) with 50 psi of 0, and (b) with no O2present in the helium seed gas. Six hundred psi total backing pressure was used In the expansions through a 330-pm orifice.
formulae were resolved by recording the spectra of both protonated benzene and perdeuteriobenzene, observing the mass shifts for corresponding peaks. None of the peaks in the mass spectrum show any breadth beyond instrumental resolution. This eliminates the possibility of there being any ion-molecule reactions during the acceleration of the ion, which would affect the time of flight. Lack of oxygen inclusion in 249-nm-produced ions also speaks against the possibility of ionmolecule reactions outside of a cluster. It is unlikely that any of the oxygen-containing species are van der Waals combinations with the parent benzene. van der Waals molecules of hydrocarbons with diatomics should not ordinarily survive the ionization process unless great care is used not to give the system any excess energy.l This is demonstrated by the fact that no oxygen benzene clusters are seen with 249-nm radiation. It is possible, however, that the species containing three or four oxygen atoms have one oxygen molecule van der Waals bound since the polarizability of the parent will be enhanced by oxygen inclusion. The power dependences of a few of the lower masses, including the benzene parent peak and C6H60, were measured. It was found that with 193-nm excitation these mass signals have very high orders of nonlinearity, up to 8 to 10, which are nonreproducible, depending in a very sensitive way on molecular beam conditions. For any given conditions the C6H60 peak usually has a higher order than the c&6 peak. Using very dilute beam conditions (cooling the benzene to dry ice temperature in the inlet chamber) such that no clusters are formed and using 249-nm excitation gave an order of nonlinearity of about two, which is to be expected for a simple two-photon ionization (the ac Stark effectg is not expected to be important in the system). From these experiments we conclude that all of the lower masses under our standard beam conditions are coming from the breakup of large clusters, a process that takes (9)
C. E. Otis and P.M. Johnson, Chem. Phys. Lett., 83, 73
(1981).
species
mass
species
C,D,
72 66 82 C6DS 77 C6HS 86 C,D,O 81 C.H.0 88 ClDiO 82 CIH60 90 C,D70 100 C6D60 94 C6H60 102 C6D7O 114 C6DSOZ 109 C6HS02 116 C,D,O, 110 C6H602 118 C,D,O, 111 C&O, 120 C,D*OZ 112 C6H802 a Most of these masses repeat themselves in the clusters, Le., each mass plus (C6H6), or (C,D6), is also seen. These peaks are seen without the presence of oxygen but are enhanced significantly upon its addition.
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many photons. Several other substituted benzenes, including chlorobenzene, mesitylene, and benzaldehyde, were tested to see if the same reaction with oxygen would take place. None showed any oxygen inclusion, possibly because other intramolecular decay channels degraded the input energy before any oxygen-related reaction could take place. In order to determine whether the initial photochemical step leading to oxygen-containingspecies resulted in ionic or neutral molecules, we performed a two-laser experiment. So that reaction products would be created, a 193-nm laser was fired longitudinally down the molecular beam. One hundred microseconds later, a 249-nm beam was introduced transverse to the beam. By itself the (relatively weak) 249-nm light produced only a small signal containing no oxygen-related masses; but with the prefiring of the ArF laser very large signals were seen which contained masses corresponding to oxygen inclusion. At the present time it is not clear whether the general increase in signal is due to long-lived metastables of some sort or due to the breakup of very large clusters which we do not ordinarily detect. Oxygen-containing products have been observed to be as much as 15% of the parent mass peak following the two laser pulses. Since all ions produced by the 194-nm pulse are swept out of the molecular beam in less than 1 ps by the accelerating electric field, the initial reaction on the way to the oxygen adduct must produce a neutral. This experiment does not answer the question of whether the 194-nm light produces a neutral oxygen-containingproduct or a long-lived neutral hydrocarbon species which subsequently reacts when irradiated by 249-nm light. Since the diatomics are weakly bound (for example, ionization by 249 nm strips the clusters of all oxygen atoms, and clusters with large numbers of bound O2are neuer seen) it is unlikely that oxygen molecules would stay bound for 100 ps to a cluster which has undergone photochemistry not involving the inclusion of 02.Thus we think the former route has more merit. Experiments in which the second laser is tunable may be able to reveal something of the nature of the initial photoproduct. The result that an initial photoreaction results in a neutral product is consistent with two-photon photoelec-
The Journal of Physical Chemlstry, Vol. 86, No. 23, 1982 4469
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tron spectroscopy experiments on benzene with 194 nm,lo in which it is seen that radiationless transitions from the lBlu state can compete successfully with ionization at low laser powers.
Discussion There is a fair amount of information about the basic reactions of oxygen with gaseous benzene. Most of this concerns the reactions of atoms since they are more reactive. It is generally acknowledged that the primary atomic process is oxygen attachment to the ring in some manner, followed by an isomerism to the most thermodynamically stable species, phenol. Loss of carbon monoxide to give C6H5has been shown not to be a primary process in beam collision of 0. benzene, but it is formed in the ionization process in the mass spectrometer detec-
+
In experiments using only 194 nm we also see processes occurring which result in products formed with the same masses as the atom-benzene work, but they are accompanied by a rich variety of reactions arising from multiple interactions and do not necessarily arise from atomic reactions. The simplest of these is the inclusion of both atoms of oxygen into the benzene to form C6H602. The structure of this species is not known, but the most thermodynamically stable form would be C6H4(OH)2. The species C6H603 and C6H604 could be either molecules such as C6H3(OH)3 or van der Waals molecules such as C6H5(OH).O2.Due to the massive energy input into a cluster in order to make these species, it is unlikely a diatomic could remain in a van der Waals molecule, which would favor the covalent molecular structure. Other smaller peaks appear in the mass spectra which also require a mechanism involving several molecules. Examples of these are C6H70,C6H80,etc. where the aromatic ring has been partially hydrogenated (it may not still be a ring). Since C+ is one of the major peaks in most spectra, there are certainly hydrogen atoms available for (10) J. T.Meek, R. K.Jones, and J. P. b i l l y , J. Chem. Phys., 73,3503 (1980).
this type of reaction. The detailed mechanism for formation of these species may be very complicated, however. Long-lived stable complexes of oxygen with benzene have been reported to be formed when liquid benzene is pressurized with about 100 atm of oxygen for several hours." If this type of complex formation were also occurring under our less stringent experimental conditions, we would probably not be able to distinguish between that and cluster formation. Since under high pressures only about 1% of the benzene molecules participate in the long-lived complexes," the substantial yield of reaction product in our experiment would seem unlikely to result from them unless they had a much higher reaction cross section than a noncomplexed cluster. This interesting complex should be retained as a possibility in these studies, however.
Conclusion Available evidence for the photochemical reaction of benzene with oxygen in van der Waals clusters indicates that benzene is acting as an antenna in absorbing the 193-nm light. A long-lived neutral photoproduct or metastable is produced which gives rise to oxygen-containing ions upon further interaction with 249-nm radiation. The primary reaction is taking place in relatively large clusters, with secondary pathways of hydrogen extraction also existing. So far we have found benzene to be unique in undergoing these intramolecular reactions. Further work is needed to identify specific reaction pathways and products. The finite nature of this system and the versatile method of product detection promise to allow greater understanding of condensed-phase photochemical reactions, studied in the gas phase.
Acknowledgment. This work has been supported by the Department of Energy under Contract DE-AC0280ER10660. Equipment was provided by the National Science Foundation. ~~
(11) G.-E. Khalil and M. Kasha, Photochem. Photobiol., 28,435 (1978).
