J. Phys. Chem. 1993,97, 8378-8383
ARTICLES Subpicosecond Transient Absorption Study of the UV Two-Photon Excitation of Liquid Alkanes Martin U. Sander, Uwe Brummund, Klaus Luther, and Jiirgen Troe’ Insritut f i r Physikalische Chemie, Universitat Gbttingen, Tammannstrasse 6, 0-37077 Gbttingen, Germany Received: March I O , 1993; In Final Form: May 24, 1993
Liquid n-alkanes (from CSto Cia), as well as selected cyclic and branched alkanes, were irradiated with 0.4-ps pulses of 10 GW/cm2 intensity at 248.5 nm. Transient absorption signals at 497 nm were recorded in pumpand-probe experiments. The observed time profiles are attributed to the result of UV two-photon excitation. The properties of these profiles appear incompatible with the assumption of ultrafast (subpicosecond) recombination of geminate electron-ion pairs. Instead, slower time scales for geminate electron-hole recombination are determined, such as those derived from pulse radiolysis scavenger studies. An ultrafast component of the absorption decay is attributed to fragmentation from higher excited singlet states, which competes with internal conversion into the SIstate of the parent molecules.
1. Introduction Over the past 40 years, great effort has been devoted to the unraveling of the physical and chemical processes initiated by high-energy irradiation of liquid Rate coefficients and reaction yields of a large number of species, encountered under radiolysis conditions, are available in tabulated form.6 Unfortunately, electron pulse radiolysis, the principal method employed in these studies, suffers from basic limitations: the time resolution cannot be extended below the’picosecond barrier” near 10 ps.7 Thus, only the longer-lived of the species initially produced are directly observable; extrapolation down to shorter time scales was only achievable by scavenger studies.* Furthermore, energy absorption with a broad maximum between 16 and 20 eV9JOand possibly “exciton splitting”p r m a 1 1 - 1 3in radiolysis lead to very broad distributions of excited species. Far better time resolution, extending into the femtosecond regime,can be obtained by fast laser flash photolysis. Moreover, two-photon UV laser excitation allows for the preparation of well-defined narrow distributions of excited species. Photolysis studies at lower time resolution always agreed well with radiolysis results, but the first laser studies with subpicosecond time res01ution~~J~ seemed to invalidate the results of earlier pulse radiolysis studies about the rates of geminate electron-hole recombination in a l k a n e ~ : ~ Jone ” ~ ~order of magnitude faster recombination rates were postulated. In order to resolve this conflict, we have undertaken a detailed UV laser irradiation study of liquid n-alkanes (from Cs to C16) as well as of selected cyclic and branched alkanes. Subpicosecond UV laser pulses were employed which, by two-photon absorption, lead to excitation energies of 10eV. The present article, extending our earlier reports,20 gives a detailed description of our results and discusses the mechanism proposed to explain the data. It is suggested that the ultrafast components in the experimental absorption signals, which have earlier been attributed to electronhole recombination on the subpicosecond time scale14J5 more probably correspond to fragmentation and electronic transition processes of higher excited states. In contrast to electron pulse radiolysis or y-ray excitation, ions apparently play only a minor role in UV photoexcitation experiments,up to excitationenergies of at least 10 eV. 0022-3654/93/2097-8378$04.00/0
2. Experimental Technique Pumpand-probe measurements were performed with a cascade dye laser system pumped by a XeCl excimer laser (Lambda Physik EMG 101 MSC). The first stage of the system consisted of a double-resonatordye laserZl(5 mMp-terphenyl in cyclohexane). The windows of a transversally pumped 5-mm cell formed a highloss resonator. Directly after the first spike of the beginning relaxation oscillations, the emission of this short resonator was quenched by the action of a prasitic half-resonator formed by a slightly tilted high-reflectingmirror set at a distance of 20 mm from the dye cell. The 140-ps pulses at 340 nm thus obtained were immediately used to longitudinallypump a short cavity dye laserz2( 0 5 ” path length, 10 mM butyl-PBD in isopropanol) consisting of a dichroic entrance mirror, which transmitted the pump pulse and reflected the 365-nm light generated,and another dichroic mirror at the exit side, which back-reflected the pump pulse and transmitted the 365-nm output. After one amplifier stage, a pulse energy of 12 gJ was obtained. Pulse durations of about 14 ps have been measured under similar conditions.22 The amplified pulses themselves were used to pump a third dye laser (1 mM BBOT in isopropanol) of the short-cavity type just described, but this time it was not the laser emission generated but rather the residual transmitted pump pulse that was of interest. The rise time of its leading edge was shortened by the laser medium at first acting as a saturable absorber, and trailing components were “consumed” by the laser action setting in immediatelyafter saturation was achieved. Thereby, a time window of ca. 8 ps (with the present design and dye concentration) was defined by this Gated Saturable Absorber (GSA).23 After amplification,a pulse energy of 3-4 pJ was achieved. The obtained pulses were exactly suited to pump the central part of our laser system, a Distributed Feedback Dye Laser (DFDL). As for the parts described above, also for this stage, we chose a design developed by Szatmlri and S ~ h l f e r .The ~~ GSA output was expanded in the horizontal plane and focused by a cylinder telescope through an amplitudetransmission grating into thenearest focal plane of a microscopeobjective. Thezerothorder beam diffracted by the grating was blocked, leaving only the beams of plus and minus first order to enter the microscope objective, from which they emerged under a certain angle into a dye cell with a front window of 0.17-mm thickness.25 In the dye solution (10 mM Coumarin 307 in DMF), a high-visibility image of the transmission grating was formed through the Q 1993 American Chemical Society
UV Two-Photon Excitation of Liquid Alkanes interference of the two beams. The quality of this interference pattern is crucial for the evolution of subpicosecond pulses in DFDLs. Carefully adjusting the energy of the incident 365-nm light by means of a horizontal slit, the DFDL was operated slightly more than 2 times above threshold. The 497-nm pulses thus generated exhibited a spectral widthof roughly0.5 nm. Anenergy of 100-150 pJ was obtained by four-stage amplification of the green pulses. The autocorrelation width of 0.7 ps (corresponding to 0.4-ps Gaussian pulses) showed that the (unamplified) DFDL output was nearly Fourier-limited. Frequency doubling of the amplified DFDL pulses in a 1-mm BBO crystal and single-pass amplification in a KrF excimer laser (a second Lambda Physik EMG 101 MSC) yielded 1 mJ, 0.4-ps pump pulses at 248.5 nm. Further operational details about the laser system may be found elsewhere.20.26 The pump pulses at 248.5 nm were slightly focused, so as to obtain an intensity of about 10GW/cm2in the sample. Transient absorptions were probed using a time-delayed 10-pJ part of the unconverted 497-nm light. Approximately 10 cm before the sample, 4% of the probe light was coupled out to serve as a reference. In the experiments described in this article, for each delay time, up to 150 shots were recorded, along with an equal number of "zero line" shots with the pump pulse blocked off. Shots were discarded, for which either the pump or the probe energy did not fall within a f 12%limit around preset values. The samples were deaerated by bubbling with argon and flowed through a 1-mm Suprasil quartz cell. On the short time scale investigated, neither impurities nor dissolved oxygen seemed to influence the signals obtained.
3. Results Figure 1 shows two representative absorption-time profiles recorded after UV irradiation of n-alkanes. The signal of n-pentane is characteristic for shorter-chain behavior (C,,, n 5 7), while the n-dodecane signal is typical for the longer-chain alkanes (n 1 8). The n-alkane series of experimental signals is displayed in Figure 2, where the members Cg to C11 have been omitted for clarity-they are similar to each other and intermediate between CF, and C12. (The members C13 to CIShave not been investigated.) The rise time of the absorption signals always corresponds to the length of the 0.4-ps pump and probe pulses. For most n-alkanes, three components of the kinetics, after the initial rise of absorption, are distinguishable: a first, fast decay (71 = 0.71.4 ps) is very pronounced in the shorter-chain alkanes, but far less marked in longer-chainalkanes. The initial decay is followed by a slower re-rise of absorption (72 = 12-22 ps) for alkanes longer than CI, in contrast to a further decay for CSand c6 (72 = 5 and 10 ps, respectively). Finally, as a third component, there is a slow decay of the absorption practically back to the zero line, with time constants ranging from 7 3 = 0.17 ns (for C5) to > l ns for larger n-alkanes (n 1 8). As an example of branched alkanes, Figure 3 shows an experimental signal for 2,2,4-trimethylpentane(isooctane). Here, the 7 2 component cannot be isolated, and the final decay, back to the zero line, is much faster (73 = 38 ps) than that observed for straight-chain alkanes. Absorption signals for three cyclic alkanes are shown in Figure 4. Despite their structural similarities, these cycloalkanesdisplay greatly differing behavior. The initial absorption spike in cyclopentane decays so fast (71 < 0.4 ps) that it cannot be fully resolved within our time resolution. A special case seems to be cyclohexane, where our measurements confirm the existence of two slow decay components, such as noticed earlier by Miyasaka and Mataga.2' For the first of these decay times, both our study and their study yield a value of =lo0 ps; for the second one, we can only give a lower limit of 0.3 ns, which is consistent with Miyasaka and Mataga's value of =l ns. In the case of trans-
The Journal of Physical Chemistry, Vol. 97, No. 32, I993 8379 t
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decahydronaphthalene (trans-decalin), the initial decay is fast (71 < 0.4 ps) but not very pronounced, such that the residual absorption after =20 ps is comparably large. Our present data are still of limited accuracy, such that it seems appropriate to stay within a simple multiexponential analysis. The time constants thus determined are summarized in Table I and compared with literaturevalues as far as available. In the following section, a model will be developed which can account for the absorption-time profiles of our study and of previous studies. 4. Discussion 4.1. mAlkanes. The previous observations of the first, fast component of the absorption decay have been interpreted in terms of an ultrafast recombination of geminate electron-ion pairs.14J5 Accordingly, the absorption remaining after a few picoseconds wasattributed toalkaneradicalcationsafterescapeof theelectron from the mutual attraction. Within this concept, the high level of residual absorption in longer-chain alkanes was tentatively explained as being due to the low electron mobility in these liquids hindering the electron's way back to its parent cation.14 This interpretation in several ways is in conflict with results from pulse radiolysis and photolysis studies, on the basis of which, in the following, we present an alternative, unifying interpretation. Radiolytical yields Ge of free ions (i-e.,electron-ion pairs that do not undergo geminate recombination) do not decrease with increasing electron mobility pe, but, on the contrary, they rise, starting from a threshold mobility of pe = 0.4 cmz V-1 s-1.
Sander et al.
8380 The Journal of Physical Chemistry, Vol. 97, No. 32, 1993
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