Multiphoton ionization of liquid benzene: the ionization mechanism

VoIumc 66, number I

MULTIPHOTON

CHEMICAL PHYSICS LElTERS

IONIZATION

15 September 1979

OF LIQUID BENZENE: THE IONIZATION MECHANISM *

T-W_ SCOTT, A.J. TWAROWSKI and A-C. ALBRECHT Deparrmer~rof Chenlisrq-. CorrzeiI U..UIerxip. Irhca. A-c-w York I-2853. us.4

Received4 June 1979

The multiphoton ionization of liquid benzene at 355 nm is found to be a three-photon process iavol~iag a tv.o-photon produced intermediate state which hves 10 ns or lonser_ Polarization experiments can be rationalized if the tuo-pboton absorption at 177 nm (6.99 cV) involves transitions into a mixture of fmal molecular states having XIg. EIg and/or Ezg symmetrics.

I_ Iutroduction Multiphoton absorption spectroscopy of benzene has been observed using several techniques_ Absorption into electronic states above the first excited singlet state has been detected by multiphoton ionization in the gas phase [ 1 J and by pulsed thermal lenslng methods in the condensed phase [3] _The multiphoton ionization of liquid benzene has been reported recentIy [S], although it was not examined in detail- Ultraviolet puIsed thermal lensing experiments in our Iaboratory have led us to a careful examination of the multiphoton ionization process in liquid benzene, and this forms the subject of the present report.

the appearance of current at the input terminal of an electrometer (Keithley 602) after the purified liquid ln the conductivity cell is illuminated by a pulse of light at 355 nm (3.49 eV). The S ns, 2 Hz light pulses are from the third harmonic of a Nd : YAG laser (Quanta Ray)_ A pinhole isolates a fraction of the laser beam which then passes through a 50 cm focal length lens before entering the conductivity cell perpendicularly to the applied electric field. The entire time profile of the photocurrent is stored digitally, averaged over many events @IO’), and then integrated to obtain the average number of charges, Q, produced per pulse (a Tektronix digital processing oscilloscope is used)_ As few as IO4 charges or currents of 5 fA can be seen in this fashion.

2_ Experimental 3_ Results and discussion Spectroanalyzed benzene (Fisher Sci. Co.) is purifled for photoconductivity measurements by storage over 5A moiecular sieves for at Ieast one week, followed by fractional distillation_ The purified liquid is then piaced in a conductivity cell having an optical path length of I cm. Two prtraIIe1 phte electrodes, constructed of polished stainless steel, supply an electric field of 8 kV/cm_ The multiphoton ionization is detected by

The multiphoton ionization signal, Q, for liquid benzene (believed to be free of ionizable impurities) is found to depend on the cube of the light intensity, I. However, many samples exhibit an intensity dependence of the form Q =A?

University.

(I)

A and B are coefficients describing simultaneous ionizations wftich depend quadratically and cubically on light intensity_ These two parameters are seen to vary with the strength of the applied electric field and

where * This work was supported by a gxmt from the National In&tote of Health and the Materials Science Center of Cornell

+B13,

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CHEMICAL PHYSICS LETTERS

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NC ai 0

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IO I

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tMICROXXJLES)

Fii. 1. The intensity dependence (Q/I’ versus f) of the photo. iontition of liquid benzene by line&y potied aht (upper curvz) and circukuly pokuized light (Iowx arve). Each point represents the zwer~ of I28 event= Each solid line shows the best tit to the kearized form of eq_ (l)_ The svera=e J.aser energy per putse (in 4) is used in p&e of the true Iaser intensity_

the temperature of the liquid_ In addition, we find that B is sensitive EO the polarization of the Iight beam, VvhiIeA is not_ Plots of Q/I’ versus I for a sanlpIe which e.xhibits both quadratic and cubic behavior are given in fig_ I for linearly and circuhuly pokuized light_ The data fit eq_ (1) over a nearly three orders of magnitude change in Q_ Since tile quadmtic component of Q (represented by the intercept in fig. 1) varies irre&uiy from sample to sample and in sevem1 cases is absent entirely, it rtppears not to retfiect an intrinsic property

of benzene_ In fig_ 2, the linearized

intensity

anaIysis is shown for spectroatxdyzed benzene with and without the purificztion procedure described

above. The slopes of the lines, which are determined by rhe ionization process \vhich is cubic in intensity, are identicaLverifying that both samples are studied under the same experimental conditions_ HGWeVer, tile untreated liquid shows rt large intercept which is absent in

the

purified

can be rationalized 2

sample. The non-zero

as the photoionization

intercept of impurity

Fii- 1. The intensity dependence (Q/I’ versus/) of the multiphoton ionbrjtion of untreated benzene (upper curve) and pusified benzene (Eower curve). Photocurrent pulses are wxaged 64 times. For each sampIe, a soIId line shows the best fit to eq_ (1).

moIecules. Absorption of one photon at 355 nnl popuIates ;1 real excited state of the inlpurity as the initial step in a sequentia! two-photon ionization. This simple scheme aIso accounts for the lack of dependence of the quadmtic rerm on the polarization of light, since the reaI intermediate state is likely to fully randoznize on the tinle scaIe of the light pulse (A~~~/~ti~~ was nlerrsured 3s 097 + 0.09). It is noteworthy that an irllpurity concentrstion ns low as IO-f2 mole Q-l can account for tile two-photon signal seen in these experiments (provided the ionization pamnleters are favomble) and that contaminants are therefore IikeIy to be a general probIem in the near ultraviolet muitiphoton ionization

of organic liquids.

The pflotoionization

of liquid benzene, which at 355 nm is cubic in light intensity, can be rationalized by tile following model_ A two-photon absorption event forms an excited singlet state (or states) at 6.99 eV which, as intensity studies indicate, does not ionize to a significant extent (the ionization threshold of benzene in condensed phase is believed to lie between

VoIume 66, number 1

CHEMICAL PHYSICS LETTERS

7-O and 7.6 eV j4,5])_ Electronic re!axation then produces a relativeIy long lived species which can ionize upon absorption of a third photon_ The existence of ;1 long-lived excited state in benzene which absorbs in the vicinity of 355 nm, has been demonstrated by excited state absorption studies [6] _ From 450 nm into the near ultraviolet, a strong and uniform excited state absorption is seen. It was suzested that this absorption corresponds to excitation from the transienr excited state into an ionization continuum_ Similar absorption transients have been seen following the two-photon excitation of neat benzene at 347 mn [7] _ Temperature studies indicate that this absorption is due to the IB,, excimer state [S] _ Unfortunately, the temperature dependence of the muitiphoton ionization signal cannot directly test whether or not the escimer is invoIved in the ionization process, since the seminate ion pairs which precede free ion formation xc themselves ionized effectively by heat (studies of the dependence of Q upon the strength of the apphed electric field indicate that geminate pairs are formed in these experiments). Similarly, the use of dilution studies to test for escimer participation is compliated by the dependence of geminate pair dissociation on the type of solvent [91 and the solvent concentration [ 1 O] _ The participarion of some long-lived intermediate state in the three-photon ionization process can be explored however by an experiment in which two purses of 355 nm light, separated in time by 17 ns, are used to generate charge_ The two p&es are S ns long (fwllm) and have negligible overlap in time. First, measurements are made of the charge_ Q1 _ produced by the initial pulse aIone, and then the delayed ~~1st alone, Q, _ Then the signs& &. with both beams coincident in space (but delayed in time) is measured. Provided oxygen is removed from the smipIe (by nitrogen bubbIing), Q1,2 is found to be 26% greater than the sum of Qr and Qa (see fig_ 3). The presence of ;t long-lived intermediate state, which does not fully deL;ly in I7 ns, is seen to enhnce the yield of free charge for the dehyed puIse_ ApparentIy upon adding osygen the lifetime of the intermediate state is shortened, for tjie number of charges formed by each puke is reduced and Q1_? is now equal to the sum of Q1 plus Q? to within 1% (this also verifies that there is no overIap in time between the two puIses)_ The long-liked intermediate responsible for three-photon ionization is IikeIy the same one seen in transient

0

15 September 1979

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Fig. 3. Photocurrent versus time for the ioniznrion of liquid benzene by two l@,hr pulses separated by a I7 ns delay- The two snxdIest curxes (nearly the same magirude) show the photocurrent due to the initial p&e alone and the delayed pulse aIone; the next larger waveform is the calculated sum of thrze two_ l-be 1Ges.t c-e shows the obsened photocurrent ashen both pulses illuminate the sxnple_ I&h sigxxl is averaged 123 times and nilrosen is bubbled inro tile conductikity cell for 5 min prior to nuking these measurements.

absorption studies. llowever, it is possible that some other exclted state. hd\ing a relatively weak absorption At 355 nm but a favorabIe ionizxion cross section_ might be the dominant intermediate. Concerning the effects of the polarization of light on the photoionization of benzene, we note rhat the three-photon coefficient, B, shouId vary with poiarization only throu$ the initial two-phoron process_ Photo-selection memory for the long-lived intermediate wili have been Iost prior to the absorption of a third photon_ From plors like those in fig_ 1 we find B drcd.JBllnear = 0.7s t 0.07. The final stat+ in an allowed two-photon CdipoIe-dipole) transition in benzene using identical energy photons must have E,,, Ezg or Al,, symmetry [I 1J_ An E,, or ET0 state ~111 give 3 polakition ratio (circular/&ear) 3 l-5, whereas an A,, f-in31state may have a maximum polarization

ratio of 0.67

in the D611 point group (this

occurs when all virtusi intermediate

states are of A,, symmetry). Since both A-,, and E,, intermedktte states are expected to make significant contributions 3

Volume 66, number I

CHEMICAL

PHYSICS JxTrERS

to the two-photon transition tensor [I&13], the poiarization ratio for an Alg final state will be considerably iess than 0.67_ The measured value of 038 therefore requires a mixture of 0verIapping moIecuIar states at 699 eV having A,, as well as Ep and/or Elp symmetry_ It is premature to speculate further about the identities of the excited states reached in benzene

by the direct absorption of two 355 nm photons_ We are currently planning more detailed polarization studies of the near ultraviolet multiphoton absorption and ionization spectra of liquid benzene. White photoionization and thermal Iensing techniques may prove usefu1 for obtaining the multiphoton absorption spectra of liquids. one might take an alternate view and consider that mu!tiphoton absorption offers a convenient method of popuIating highly excited states in the interior of a liquid, and that multiphoton ionization provides a means of studying the photophysics of these states_ For instance, one can hope to locate accurately the ionization potential of pure Iiquids by tuning the incident beam until a threshold for two-photon ionization is reached_ In another application, we are currentIy deveIoping an optical method to directly measure the mobilities of both positive and negative charge carriers produced in buik Iiquids by muItiphoton ionization. Studies of excitonic migration of highly exited states in liquids arc also possible_ Finally, it appears that multiphoton

15 September 1979

ionization can offer a remarkably sensitive test for impurities in liquids, provided their ionization thresho!d lies below that of the host material_

References [I] P.M. Johnson,J. Chem. Phys. 64 (1976) 4143. [21 AJ_ Twarowski and D-S_ KIiger, Chem. Phys. 20 (1977) 25% [3] V_ Vaida, M-B. Robin and N-A. Kuebler, Chem. Phys. Letters 58 (1978) 557. [4] C. Fuchs, F_ Heisel and R. VoItz, J. Phys. Chem. 76 (1972) 3867. [S] U_ Asafand LT. Steinberger. Chem- Phys. Letters 33 (1975) 563s [6J R_ Bonneau end J_ Joussot-Dubien. Chem. Phyr Letters 3 (1969) 353_ [71 J-T_ Richards and JX_ Thomas. Chem_ Phys Letters 5 (1970) 527. [SI R_V_B ensasson. J-T. Richards and J.R. Thomas, Cbem. Phys. Letters 9 (197 1) 13. [9] R-A_ Holroyd and R-L_ Russell. J. Phys. Chem. 78 (1974) 2128. [ 101 PM_ Borsenberger. L-E_ Contois and D-C_ Hoesterey, J. Chem_ Phys. 68 (1978) 637_ [ 111 WM. hfcCIain, J_ Chem- Phys 55 (1971) 2789. [ 121 D.&l_ Friedrich and W-M_ McCktin. Chem- Phys Letters 32 (1975) 541. [131 GM_ Korenowti. L-D_ ZiqIer and A-C_ AIbrecht, J_ Chem- Phys- 68 (1978) 124%