Collisionless relaxation of photoexcited benzene ions

Volume

125, number

5,6

CHEMICAL

COLLISIONLESS

RELAXATION

PHYSICS

18 April 1986

LETTERS

OF PHOTOEXCITED

BENZENE

IONS

Robert C. DUNBAR Chemrstr3, Deporrment, Received

6 January

Case Western Reseme

llmuersti~v, Cleoelund, OH 44106. USA

1986; in final form 24 January

1986

The colhsionless energy relaxation of benzene ions followmg excitation by a visible-wavelength photon has been measured by kmetlc analysis of two-photon photodissociatlon. usmg a chopped laser at varying choppmg rate irradiating ions in the ICR ion trap. At 488 nm the relaxation rate of the ion of 11 s-’ is faster by a factor of 4 or 5 than that calculated for neutral benzene. The relaxation rate varies with wavelength over the range 458-515 nm from 8.5 to 12.3 SK’ in the manner expected for Infrared-fluorescent energy dissipation from an excited ion whose excess energy is stored as vibratlonal excitation.

1 . Introduction

Ion trapping methods make possible the study of processes occurring in isolated molecules on a time scale of seconds or longer, Collisionless dissipation of excess internal energy is often very slow in polyatomic molecules, and is a particularly interesting subject for study by these new techniques. Study of several polyatomic ions in a variety of experiments [l-3] has shown that the time required to dissipate one or two eV of internal energy ranges from tens to hundreds of milliseconds. It is commonly assumed that the mechanism of relaxation is infrared fluorescence, which is expected to show relaxation rates in this range. Dissipation of internal energy following photoexcitation plays a central role in the sequential twophoton photodissociation kinetics fust described in benzene ions by Freiser and Beauchamp [4], Following absorption of the first photon, dissociation will occur only if a second photon is absorbed before the ion has had time to dissipate sufficient internal energy to bring it below the one-photon threshold. Characterizing the competition between dissociation and relaxation has been found to be a versatile approach to studying both collisional and collisionless relaxation processes [S] . In benzene, van Velzen and van der Hart [l ] used an extrapolation of continuous-h-radiation results to zero pressure to report a zero-pressure relaxation rate of 8 s-l at 488 run. Freiser and Beauchamp’s original suggestion [4] was 0 009-2614/86/$03.50 (North-Holland

0 Elsevier Science Publishers B.V.

Physics Publishing Division)

that the energy of the fast photon is rapidly converted to, and stored as, vibrational excitation in benzene ion. ‘Themost plausible alternative for energy storage for ~100 ms would be in a spinexcited species (e.g. a quartet); against this it has been argued [6] that the lowest quartet lies above the first excited doublet and would not be long-lived. There might also be a possibility of energy storage in a structural isomer. Circumstantial arguments in favor of vibrational energy storage are the fact that observed collisionless relaxation rates are of the right order of magnitude; and the observation that the sequential two-photon mechanism is widely observed in a variety of ions so that no unique attribute of benzene is involved. However, in the absence of a direct observation of fluorescence, vibrational energy storage remains only the most likely of the alternatives, and more evidence is needed. One specific and testable prediction from the vibrational energy-storage mechanism is the wavelength dependence of the rate of radiative relaxation to an energy below the one-photon threshold, which is predicted to become slower with increasing photon energy. Fig. 1 illustrates on an energy diagram the relaxation mechanism and its dependence on wavelength. Other energy storage modes most plausibly imply no wavelength dependence, or faster relaxation at shorter wavelengths. The present experiments were undertaken first to measure the low-pressure relaxation rate in benzene ion by a reliable technique, and second to test the predicted wavelength dependence. 543

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Volume 125, number 5,6

18 April 1986

2. Experimed 4

Dissociation

1

Threshold

The ICR ion-trapping instrument, the argon-ion

4

1

hv,

h

515 0’

I

458

nm

nm

I

Fig. 1. An energy picture of two-photon photodissociation of benzene ions at two different wavelengths. The arrows marked hq and huz represent the energy increments from the first and second photons, and the dashed AE arrows represent the energy to be dissipated in relaxation processes to bring the ions below the one-photon threshold. The ions start with thermal energy of about 0.1 eV and dissociate when they exceed 4.04 eV of internal energy (the dissociation energetics are derived from the thermochemical data of ref. [7] ). The relaxation rate is governed by the time needed to dissipate energy AE.

Although low-pressure relaxation rates have been measured using both continuous irradiation [1 ] and also two-pulse irradiation [8] , we have adopted a repetitively pulsed irradiation scheme [2 91 as giving the best combination of sensitivity and accuracy. The laser is chopped at a varying rate, in such a way that the average power level is constant. Variations in observed photodissociation with chopping rate are solely due to the varying ability of the ions to relax between successive pulses. Simple consideration indicates that slower pulse rates give a greater extent of dissociation. More careful analysis indicates that, as the pulse rate slows from high rates, the pulse rate at which an upward trend in the extent of dissociation becomes apparent is roughly half the relaxation rate [2] . For quantitative relaxation-rate determinations, we fit the chopper-rate dependence to a complete computer simulation of the two-photon kinetics, varying the relaxation rate for best fit. We have confidence in the absolute relax&ion rate values, since the kinetic time base in this technique is determined accurately from the known chopping rates. 544

laser and the chopping procedure were as used in previous work [2,9] . Benzene ions were generated at nominal electron energy around 12 eV, and pressures indicated at the ionization gauge were 2 or 3 X lo-* Torr (real pressures [lo] below 10e8 Torr). The time between charge-transfer collisions was thus more than 2 s, making collisional relaxation completely negligible compared with collisionless relaxation. The ions were trapped and irradiated for 3 s. In order to avoid any possible problems with regional depletion of the ion population the extent of dissociation was limited to 5 10%. The experiments used the laser beam spread to a diameter somewhat smaller than the ICR cell aperture. However, to check for possible artifacts from the inhomogeneous illumination, an experiment was done comparing dissociation with the laser chopped at 2 Hz with that with continuous irradiation through a 1/lo neutral density filter, using the multiple-mirror beam homogenizer which has been described [ 111. The results were consistent with the rest, indicating no major inhomogeneity artifact. The three wavelengths used were 5145,488 and 458 nm. The nwnerical simulation program used to derive relaxation rates from chopper-rate dependences has been discussed [9] . The simulation confums the observations that the chopper-rate dependence is very insensitive to light intensity I, and that the extent of dissociation is accurately proportional to Z2 at 510% dissociation. Small light intensity variations were corrected using an Z2 correction. Several studies [ 1,121 have suggested substantial differences in photoexcitation rates for the fust and second photons of the two-photon sequence. The simulation calculations indicate that the relaxation rate derived from chopper-rate-dependence data is very insensitive to the ratio of the two photoexcitation cross sections, depending only on their product. Thus the suggestion of van Velzen and van der Hart [l] that the ratio in benzene ion is about 4 is not expected to have any significant effect on the precision of the present determinations.

3. Results and discussion Fig. 2 shows the chopper-rate dependence for the benzene ion photodissociation, and it is seen that the chopper effect is large. The solid line in fig. 2 is calculated for a relaxation rate of 12 s-I, which gives a very acceptable fit to the observed curve shape. There are small differences in the data for the three wavelengths: The 458 nm points level off the fastest and at the highest value, corresponding, as expected, to the slowest relaxation. Non-linear least-squares fitting of the three different wavelength data sets to the numerical simulation calculation yielded statistically significant differences in the relaxation rates and gave values in accord with the more precise measurements described below. However, determining the extent of BENZENE

0

18 April 1986

CHEMICAL PHYSICS LETTERS

Vohtme 125. number 5,6

ION PHOTODlSSOClATlON

A

515 nm

0

488 nm

m

458 nm

1”’

I

I

1234

8

12

Chopper

Rate

wavelength variation of the relaxation rate between these three wavelengths is obviously near the limit of precision of this data set. To obtain more precise comparisons of relaxation between the three wavelengths, a direct ~ompa~son was made of dissociation with 2 Hz chopped illumination versus that with continuous irradiation using the l/l0 neutral density filter. To make the data as precisely comparable as possible, the laser intensity was adjusted to give the same extent of dissociation at each wa~len~h (about 10% dissociation at 2 Hz). Using the numerical simulation these ratios can be converted to relaxation rates; these calculated rates are plotted in fig. 3. The statistical precision is good enough to show clearly the variation in relaxation rate with wavelength. The value of 11 s-l for ~o~~o~e~ relaxation at 488 nm is acceptably close to the value of 8-2 reported by van Velzen and van der Hart [l] . Since their value involved an extrapolation of high-pressure data, and also required the assumption that collisional quenching proceeds at the charge-transfer rate, we believe the present chopped-laser dete~ation is more definitive. If the collisionless relaxation is accepted as being

(Hz)

Fig. 2. Chopper-rate dependence of benzene ion photodissociation. The dissociation D is defined by D = -In(lightsn ion signal/light-off ion signal). The D values for 488 nm are as measured, but for easier comparison, the values at 514 mu have been scaled up by a factor of 1.5, and those at 457 mn by 2.25. The solid curve is the numerical kinetic simulation result assuming a relaxation rate of 12 s-*, and a photoexcitation rate (first- and second-photon rates assumed equal) of 2 1 s-l with a light-on duty cycle of 0.033.

Photon

Energy

(oV)

Fig. 3. Relaxation rate as a function of photon energy (points with error bars). The solid curve is calculated as described in the text.

545

Volume125, numba 5,6

CHEMICAL PHYSICSLETTERS

due to infrared fluorescent ion cooling, it is interesting to compare the ion results with the behavior expected for neutral benzene. As has been discussed [ 131, the infrared radiative properties of the neutral can be calculated with confidence from the known infrared absorption intensities, via the Einstein transition coefficients. The procedure for this, assuming free energy flow among the vibrational modes of the molecule, has been described [ 131. Using the measured IR intensities for benzene [ 141, the radiative cooling was modeled for the internal energy range from 25000 cm - ’ down to thermal *. The relaxation time as defined for the two-photon dissociation kinetics is the time required to dissipate enough of the initial photon energy so that absorption of a second photon cannot bring about dissociation, i.e. the time needed to fall below the one-photon threshold. If the benzene ions radiated at the rate calculated for neutral benzene, their relaxation rate in these experiments would be about 2.5 s- ‘. Clearly the observed rate around 11 s-L is far greater, and we conclude that collisionless relaxation of the ion is faster by a factor of 4 or 5 than expected by analogy with the neutral. This surprisingly high rate is not fully understood; the presence of the charge in the ion gives possibilities of charge displacement and enhanced’vibrational transition dipole moments which may not be present in the neutral *. The neutral molecule is clearly a poor guide to understanding the quantitative radiative rate of the ion. However, if it is granted that the ion radiates in a manner similar to the neutral, but with enhanced intensities, the wavelength dependence of the relaxation rate can be calculated and compared with experiment. It was assumed that the ion has the same vibrational frequencies as neutral benzene, but with each vibrational intensity 4.2 times higher than the corresponding neutral value. The relaxation time was then calculated at each of the three wavelengths using the normal calculation procedure. * Under conditionsof strongoptical pumpingand slowre-

laxationit could happenthat the undissociated,energyrelaxed ion population could be heated significantly above ambient temperature. Calculation with the pumping and relaxation rates of the present experiments indicates that such a heating effect is entirely negligible here, and the ion population can be modeled accurately as remaining at ambient temperature. ** This line of possibilities was pointed out by Professor J.L. Beauchamp.

546

18 April1986

The calculated relaxation rates are shown in fig. 3 as the solid curve. The fit is not perfect, but seems satisfactory in view of the uncertainty about the ion IR radiative characteristics. We conclude that, if the IR intensities for the neutral are scaled up by a factor of about 4, the IR radiative energy dissipation postulated as the collisionless relaxation mechanism gives a quantitatively satisfactory account of the wavelength dependence. ‘Ihe wavelength dependence shows the expected decrease in relaxation rate with increasing photon energy, giving additional support to the vibrational energy storage model for this two-photon dissociation.

Acknowledgement Support for this work was provided by the National Science Foundation. The ICR spectrometer was built with generous support from SOHIO. References ill P.N.T. van Velzen and W.J. van der Hart, Chem. Phys. 61 (1981) 325.

121R.C. Dunbar, J. Phys. Chem. 87 (1983) 3105. 131L.R. Thorne and J.L. Beauchamp, in: Gas-phase ion chemistry, Vol. 3, ed. M.T. Bowers, (Academic Press, New York, 1984) ch. 18; J.M. Jasinski and J.I. Brauman, J. Chem. Phys. 73 (1980) 6191; G. Caldwell and J.E. Bartmess, J. Phys. Chem. 85 (1981) 3571. 141B.S. Freiser and J.L. Beauchamp, Chem. Phys. Letters 35 (1975) 35. 151C. Dunbar, in: Gas-phase ion chemistry, Vol. 3, ed. M.T. Bowers (Academic Press, New York, 1984) ch. 20. 161J.P. Maier, in: Kinetics of ion-molecuIe reactions, ed. P. Ausloos (Plenum Press, New York, 1979) p. 437. 171J. Dannacher, H.M. Rosenstock, R. Buff, A.C. Parr, R.L. Stockbauer, R. Bombach and J.-P. Stadelmann, Chem. Phys. 75 (1983) 23. 181R.C. Dunbar and J.H. Chen, J. Phys. Chem. 88 (1984) 1401. 191N.B. Lev and R.C. Dunbar, Chem. Phys. Letters. 84 (1981) 483; J.P. Honovich, R.C. Dunbar and T. Lehman, J. Phys. Chem. 89 (1985) 2513. [lo] J.E. Bartmess and R.M. Georgiadis, Vacuum 33 (1983) 149. [ 111 R.C. Dunbar, Chem. Phys. Letters 115 (1985) 349. [ 121 T.E. Orlowski, B.S. Freiser and J.L. Bauchamp, Chem. Phys. 16 (1976) 439. (131 R.C. Dunbar, Spectrochim. Acta. 31A (1975) 797. [ 141 T.G. Goplen, D.G. Cameron and R.N. Jones, Appl. Spectry. 34 (1980) 657.