Chemical Physics 106 (1986) 171-178

North-Holland, Amsterdam

171

CONCURRENT PHOTODISSOCIATION AND MULTIPHOTON IONIZATION PROCESSES IN CH,I FROM 266-307 nm

Yifeng JIANG, Maria R. GIORGI-ARNAZZI and Richard B. BERNSTEIN

Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90024, USA

Received 22 January 1986

Using a two-color, pulsed laser, time-of-flight mass spectrometer, experiments have been carried out on CHsI over the

wavelength range 266-307 nm, with the goal of elucidating the concurrent photophysical and photochemical processes leading

to ionic products. These include (a) the one-photon excitation (A + %) followed by prompt dissociation, leading to neutral I

atoms (mainly I*, 5 *P,,*) and vibrationally excited CHs radicals, and (b) two-photon resonant excitation, plus one-photon

ionization of I and CH,. At 266 nm, even with laser pulse energies as small as 0.1 ml, weakly focused (0.3 m focal length lens)

on CH,I at 1O-6 Torr, essentially all the =104 molecules in the focal zone are dissociated, after which the I and CH, are

ionized with power law indices n = 2. By photolyzing CHsI at 266 nm as above and detecting the I* by its sharp (2+1

REMPI) resonance at 281.7 nm, essentially all the CHsI is “converted” to I+ (for pulse energies 2 0.3 ml; 0.25 m focal

length lens). The I(*Ps,*) fragment is also detected, via its 2 + 1 REMPI at 277.9 nm; the CH, similarly at 286.3 nm. Both

neutral photoproducts can be detected, albeit at low efficiencies, via MPI at 266 nm. From the various power-law experiments,

it is possible to distinguish among the concurrent photochemical processes in CH,I. The predominant pathway to ions is from

photodissociation followed by ionization of the neutral CH, and I photofragments.

1. Introduction

The photophysics and photochemistry of methyl iodide in the near UV have been well studied, since the first continuous absorption spectra were reported by Herzberg and Scheibe in 1929 [l] (see refs. [2-51). Photofragmentation spectroscopy ex- periments, starting from those of Riley and Wil- son in 1972 [6] (see refs. [7-91) and spectroscopic studies [lo-121 have elucidated the one-photon process:

CH,I(%) .,ay”v [CH,I(A)]

: (X,(k) + I(k,,, *b/2). (1)

Laser polarization angle-dependence measure- ments by Dzvonik et al. [13] and others [6-91 have set an upper limit on the lifetime T of the A state of the CH,I. Recently, a direct (picosecond, “real-time”) measurement by Knee et al. [14] has yielded the value r < 0.5 ps.

At high laser powers, ions are produced, so

multiphoton processes are involved [14-171. In the present study, we attempt to clarify the concurrent photodissociation and multiphoton ionization (MPI) processes in CH,I in the wavelength region 266-307 nm. It will be seen that the predominant ion is I+ from the MPI of the neutral I from the photodissociation of the CH,I. Virtually no CH,I+ is formed under the conditions of the present experiments.

There is ample evidence for neutral precursors in the MPI of many polyatomics [18], especially for weakly bound transition-metal compounds such as polynuclear metal carbonyls [19], etc.

In the time-resolved experiment of ref. [14], a molecular beam of CH31 was photolyzed with a 280 nm pulse from a picosecond laser; then (0.5 < t < 10 ps) the I and I* atoms were detected by MPI at their respective resonance wavelengths of 304.7 and 304.0 nm. Recently, VUV fluorescence has been used to study photodissociation and pho- toionization in CH,I by Tsukiyama et al. [20]. MPI of atoms has been thoroughly investigated [21]. The photoionization and MPI of atomic I has

0301-0104/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

172 Y. Jiang et al. / Photodrssociatron and multiphoton ionization III CH,I

been well studied [22]; similar but less extensive work has been reported for Br, Cl and F [23].

2. Experimental

The laser time-of-flight mass spectrometer (TOFMS) is that described previously [24], with minor modifications. The experimental arrange- ment is shown in fig. 1. The fourth harmonic of the Nd: YAG laser (Quanta Ray DCR) was the source of the 266 nm radiation. Laser pulses at 10 Hz repetition rate, of duration = 5 ns and pulse energies up to 10 mJ were obtained. The quanta Ray PDL tunable dye laser (pumped by the 532 nm output from the laser) produced pulses in the region 554-614 nm (using R-6G and sulforhoda- mine 640 dyes). This output was frequency-dou- bled by a KDP(1) crystal to produce pulses in the 277-307 nm region with energies up to = 1

MOLECULAR

LEAK ““\’

MULTIPLIER ION DRIFT TUBE (lm) DETECTOR

mJ/pulse. The laser beams were focused into the ionizing region of the TOFMS (through a fused- quartz window) by a UV-grade lens of focal length 0.25 m (for the one-color experiments). An ad- ditional lens of similar quantity, focal length = 0.3 m, was used for the 266 nm radiation. A filter (Corning 7-54) blocked the visible light. A di- chroic mirror was used to combine the dye laser output with the 266 nm beam (for the two-color experiments). The average pulse energies were measured with a calorimeter power meter (Sci- entech model 38-0105) with isoperibol enclosures (Scientech model 36-0203).

3. Results

The 70 eV electron impact (EI) mass spectrum of CH,I at 5: 5 x lop6 Torr consisted of three principal ion peaks, the parent molecular ion

/

O.sClLLOBCOPE

I t_

c D. E.C. 1 MING - 11 TERMINAL

.

Fig. 1. Schematic diagram of the apparatus (for further details, see text and ref. [24]).

Y. Jiang et al. / Photodissociation and multiphoton ionization in CH, I 173

CH:

X281.74 ““I

0.1 - 0.3 mJ

CH,I+

El 70~

L-+L- m/z

Fig. 2. REMPID mass spectrum for methyl iodide at the

indicated values of the laser wavelength and pulse energy. At the bottom is the electron impact (70 ev) mass spectrum of methyl iodide determined under the same conditions as for the

laser ionization experiments.

CH31+ (m/z = 142), the I+ (127) and CHT (15) ions, as shown in fig. 2. The laser ionization mass spectrum at A = 266 nm showed no parent ion, but both I+ and CH: ions, fig. 2, i.e., CH,I+/I+ < 0.02. This is the case even at the lowest laser pulse energies, in contrast to the situation exem- plified by ref. [17], in which two-photon reso- nances at 402, 370, 366 and 362 nm were utilized in overall three- or four-photon ionization processes to form CH,I+ as the primary ion prod- uct. The absence of parent ion in the 266 nm irradiation is not in accord with the findings of Danon et al. [15]. This represents a serious experi- mental discrepancy, which we have been unable to resolve *.

Our presumption is that the I+ is derived from the neutral I photofragment (of the one-photon photodissociation reaction), the ionization step

* Actually, the molecule studied in ref. [15] was CD,1 (whose parent ion was observed), whereas the present work deals with CH,I.

being a non-resonant MPI process. The CH: could be derived either from a direct two-photon ion-pair production step or from the ionization of the neutral CH, photofragment by non-resonant or near-resonant MPI [15-17, 25-281 **. These is- sues are clarified by the results presented below.

The key to understanding the concurrent one- photon (neutral) and multiphoton (ionization) processes in CH,I is the wavelength- and power-dependences of the I+ and CH: ion inten- sities. Here we take advantage of the known two- photon resonance in the REMPI of atomic I (both the 2P3,2 and 2P,,2 states) and the CH, radical. Since the n + (I* absorption spectrum of the CH,I is continuous over the wavelength range of the present study, any sharp enhancement of the I+ or CH: ion signals at discrete laser wavelengths is evidence for the presence of neutral precursors [15-17,19-241. Fig. 3 shows the I+ ion peaks arising from the irradiation of CH,I (by the focused beam of the dye laser) at seven wave- lengths from 298 to 307 nm. Two additional strong peaks were observed, at 281.74 and 277.86 nm, which served as indicators of the I(2P1,2) and I(2P,,2) states, respectively. In all of these cases, no detectable signal of CH,I+ or CHZ was ob- served. Fig. 2 includes an example of a laser-in- duced mass spectrum of CH,I at 281.74 nm, showing only I+ (presumably from the I(2P,,2) photofragment precursor).

When the CH,I was irradiated by a pulse at 281.74 nm in the presence of a relatively weak pulse (0.3 mJ, e.g.) at 266 nm, the I+ yield in- creased significantly over that from the 281.74 nm pulse alone; at the same time, CH: ions were formed in significant abundance. Fig. 4 shows a log-log plot of the intensity of the I+ signal (ions per pulse) as a function of the laser pulse energy at 281.74 nm and with the addition of 0.3 mJ per pulse of 266 nm radiation. There is a significant enhancement of the I+ signal due to the additional photofragmentation of the CH,I at 266 nm (where the photodissociation cross section is = 4 times that at 281.7 nm [29]).

Experiments were also carried out using EI

** See also ref. 1111.

174 Y. Jiang et al. / Photodissociation and multiphoton ionization in CH,I

a.02 “Ill -

X f nm

Fig. 3. Yield of I+ from CH,I as a function of nominal laser wavelength, showing several of the resonances for “analytical” usage.

Ion intensities relative only.

fragmentation (1 us pulses of the e--beam) to- gether with the 281.74 nm laser pulses, with differ- ent delay times. Although it was easy to resolve (temporally) the I+ derived from the laser pulse from that due to EI, there was insufficient neutral fragment number density generated from the (broadly distributed) electron beam to effect any detectable change in the focused-down laser-in- duced I+ ion signal.

The most revealing experiments were those of the laser-power dependence of the I+ ion intensi- ties (from the overall process CH,I -+ I --, I’) measured at 281.74, 277.86 and 266 nm, and of the CH: ion signals (CH,I -+ CH, -+ CH:) mea- sured at 286.32 and 266 nm [25-281. Fig. 5 sum- marizes the observations.

Based on many experiments, it is known that the I*/1 branching ratio from the photodissocia- tion of CH,I in the n + u* band strongly favors the excited spin-orbit state of the I atom. Thus the wavelength of choice for detection of the pho-

tofragmentation turns out to be 281.74 nm, corre- sponding to a high two-photon resonant cross section leading to ionization. From fig. 5 it is seen that, for low laser pulse energies (5 0.08 mJ), the I+ signal increases with a power law index n of = 3. In the range 0.08-0.28 mJ, the It intensity follows a second-order law, showing saturation behavior (n = 3/2) above 0.28 mJ (all with the same focusing lens condition). For X = 277.86 nm (a sensitive wavelength for I(’ P3,2) detection), the power law index is ca. 2 from 0.08-0.45 mJ, with no saturation yet observed.

For CH,, the two-photon resonance wave- length (X = 286.32 nm) leading to ionization was chosen as the most sensitive in the conveniently available wavelength region. Yet the CH: ion intensity is several orders of magnitude below that of the I+ ions from either of the two resonant wavelengths whose data are plotted in fig. 5. (It must be presumed that the one-photon photolysis produces an equal number of CH, and I frag-

Y. Jiang et al. / Photodissociation and multiphoton ionization in CH,I 175

- 1 , , , , ,,,,, , 0.001 O.Ol 0.1 0.3

281.7 nm PULSE ENERGY/ mJ

Fig. 4. Yield (number of ions, N, per pulse) of I+ from CHsI at a pressure of 1 x 1O-6 Torr as a function of 281.7 nm pulse

energy, open circles; closed circles denote results with 281.7

nm plus 266 nm (0.3 ml) irradiation. The lower line is drawn

with a slope of 3; the upper line, drawn with slope 2, is an

extension downward of the n = 2 behavior observed at some-

what higher pulse energies (X = 281.74 nm), fig. 5.

ments, so the small ratio of CH: to I+ is due to the smaller MPI cross section for the polyatomic CH, with its higher density of states than for I atoms.)

The CH: ion yield follows a second-order power law as expected for the presumed 2 + 1 REMPI process.

Fig. 5 also shows the power dependence of the I+ and CHT ion signals at 266 nm. Both follow second-order power laws, with some unexpected saturation for the CH, + CH: process. (At the highest powers, there may be some fragmentation of the_CH: ion.) The ratio of CHZ to I+ is somewhat smaller than that reported in ref. [15], but the experimental conditions were not precisely comparable.

Returning again to fig. 4, we note that the

USER WLSE ENERGY/ mJ

Fig. 5. Yield (number of ions, N, per pulse) of I+ and CH: as

a function of laser pulse energy (one color) for CH,I at

1~10~~ Torr. (e) I(5P5*PQ2) -+ I+, A = 281.74 nm; (0)

I(SP’*P$,) + I+, X = 281.74 nm, separate experiment, scaled

by a factor of 0.4; (0) I(5PszP+) * I+, h = 277.86 nm; (W)

CH,(% 2A’;) -+ CH;, A = 286.32 nm; (A) I(5Ps2Pp/,) -f I+, h = 266 nm; (A) CHs(% *A’;) + CH;, h = 266 nm. In order

to cover the many orders of the ordinate, it was necessary to

carry out the experiments at different pressures of CH,I,

ranging from 2~10~~ to 2x 10W5 Torr. The signals were

scaled so that the ordinate values refer to a “standard” pres-

sure of 6 x 10m6 Torr, nominal ion gauge pressure. Based on

known ion gauge sensitivities, this corresponds to a partial

pressure of 1 x 10V6 Torr. This value was used in the calcula-

tion of number density in the focal volume. The effective focal

volume for the experiments at 266 nm was smaller by a factor

of = 10 relative to that of the other wavelengths [32]. Thus to

compare yields, the N values at 266 nm need to be multiplied

by = 10.

additional photodissociation due to the added 266 nm radiation effectively extends down to lower laser power the -range of the n = 2 power law

176 Y. Jiang et al. / Photodrssocratron and multiphoton ionization in CH,I

dependence for the It intensity (I ---) If at 281.7 nm). The implications of this are discussed below.

4. Discussion

The present results are best understood by con- sideration of the key features of the energy-level diagram for the CH,I system, shown in fig. 6. For ground vibrational state CH,I, there is no two- photon resonance route to the parent ion. The I+ production at 281.7 nm is believed to be de- termined by the stepwise process

CH,I “,w CH,(ji: ‘A’;, u) + I(SP”P,“,,,,,,), (2)

followed by a fast one-photon ionization step

I(nP2S,2) hw -+ I+ + e-.

GH,I (X’N)

(4)

This is consistent with the third-order power dependence observed for pulse energies below = 0.08 mJ. For pulse energies greater than this, the photodissociation step is believed to be essentially 100% complete (effectively all the neutral CH,I molecules within the focal zone being dissociated), so the rate-determining process is the two-photon excitation of the I atoms. Then the power law becomes n = 2. For pulse energies above = 0.28 mJ, this process also has reached saturation, and essentially all of the I(*P,O,,) atoms in the focal volume have been ionized. Beyond this power level, the MPI focal volume gradually increases,

giving the standard 3/2-power law [31]. The number of I+ ions formed per pulse (7.6 x

103) at 0.28 mJ is believed to be the total number of I*(2P1,2) atoms in the MPI focal volume. This number can be estimated from the known CH,I number density and an estimate of the effective focal volume for a two-photon process [32]:

I (tip: ‘0

Fig. 6. Detailed energy-level diagram summarizing many of the processes involved in the present study on methyl iodide. Shaded zone denotes A-state (n -+ e*) region. Shown are some suggested (ref. [15]) MPI-fragmentation processes of methyl iodide. The first is a two-photon ionization (TPI) to the ion pair CH: + I-; the second, requiring some 1850 cm-’ of internal excitation of the CH,I, is a TPI process to the CHjI+ E state (and possibly a third photdn to the B state); the third is a two-photon excitation to a high Rydberg state of CH31, which can fragment to excited I atoms. The one-photon excitation (A +- %) leads to fragmentation to the 5P52P ,,2.3,2 I states and vibrationally excited CH,(%) radicals. The latter are detected via the two indicated processes, the first being a 2 + 1 REMPI at 286.3 nm, the second similarly at 266 nm; the various 2 + 1 REMPI processes for I atom detection used in the present work are indicated. The one-photon ionization of the 6s 4P5,2 and 6s 2P3,2 I atoms is also shown. Energies are taken from refs. [15,26,30].

Y. Jiang et al. / Photodissociation and multiphoton ionization in CH, I 171

v = A3f 4/2aw4, (5)

where X = 281.7 nm (wavelength), f = 0.25 m (fo- cal length), w = 3 mm (waist radius of the incident laser beam). From eq. (5) V= 1.7 X 10e4 mm3. Thus at a CH,I pressure of 1 X lop6 Torr, there are 5.5 x lo3 CH,I molecules in the focal volume. The relative quantum yield at 282 nm is I&( Z3,2 + Zi,*) = 0.8 [lo-121. Thus the estimated number of I* atoms produced per pulse is 4.4 X 103.

This value is lower by a factor of 5: 2 with respect to the experimental estimate, but the agreement is believed to be reasonable in view of the difficulty in ascertaining an accurate waist value w (which appears to the fourth power in eq. (5) for the focal volume).

The I+ production at 277.9 nm and that of CH: ion at 286.3 nm are believed to be de- termined by the stepwise processes:

CH,I “,” 277.9 nm

CH,(ji: 2A’;, u)

or 286.3 nm

+ I(5p5 2p~,2,3,~ 13 (6)

I*(5P52P$2) 2,tynmI(6P4 D30/2) 2 I+ + e-, (7)

CH,(ji 2A’;, v)~~;~“~CH,(~P~A’;)

hw + CH: + e-. (8)

Evidence for vibrationally excited CH, from (6) comes from ref. [12]. For pulse energies greater than 0.08 mJ at 277.9 and 286.3 nm, the photodis- sociation step is believed to be saturated, and the power law should be second order.

Danon et al. [15] suggest that the I+ and CH: formed at 266 nm are due to the following

processes:

2hw CH,I -+ CH; + I-, (9)

CH312~CH31(10d) -+ CH3

+I(6S2p,,,, 6S4P,,,), (IO)

I(2P3,2, “P5,z) 2 I+ + e-. (11)

However, there is no direct experimental evidence

for these processes. It is suggested that the follow- ing scheme can also explain the present experi- mental results at 266 nm (see fig. 6):

CH,I “,” CH,(ji: 2A’;, U) + I(5Ps2P;,,,3,2), 266 nm

(12)

2hw I(5P52P$2) + I(7P2 DgO/2) 2 I+ + e-, (13)

CH,(% ‘A’;, v)2yCH3(5f2E’) 2 CH: + e-.

(14)

For 266 nm pulse energies far greater than 0.08 mJ, the n = 2 power laws for I+ and CH: produc- tion are reasonable, assuming the rate-determining step to be the two-photon resonant excitations of (13) and (14).

5. Concluding remarks

The concurrent photodissociation and multi- photon ionization processes in CH,I from 266 to 307 nm have been studied. The processes include (a) the one-photon excitation (A +- 2) followed by prompt ( < 1 ps) dissociation, leading to neutral I atoms (mainly I*, 5 2P1,2) and vibrationally ex- cited CH, radicals [12], and (b) two-photon reso- nant excitation, plus one-photon ionization of I and CH,. At 266 nm, even with laser pulse en- ergies as small as 0.1 mJ, weakly focused (0.3 m focal length lens) on CH,I at low6 Torr, essen- tially all the = lo4 molecules in the focal zone are photodissociated, after which the I and CH, are ionized with a power law n = 2. By photolyzing CH,I at 266 nm as above and detecting the I* by its sharp (2 + 1 REMPI) resonance at 281.7 nm, essentially all the CH,I is “converted” to I+ (at pulse energies 2 0.3 mJ, 0.25 m focal length lens). The I(2P3,2) fragment is also detected, via its 2 + 1 REMPI at 277.9 nm; CH, similarly at 286.3 nm. Both can be detected, but at lower efficien- cies, via MPI at 266 nm.

From the results of these various power law experiments, it has been possible to clarify the concurrent photochemical and ionization processes in CH,I, over the wavelength region 266-307 nm.

178 Y. Jiang et al. / Photodissociation and multiphoton ionization in CH,I

The key result is that photodissociation into neu- tral fragments, followed by their ionization, is the dominant pathway to positive ion formation via laser irradiation in the near UV.

Note added in proof

Since the submission of this paper a somewhat relevant publication has appeared [33].

Acknowledgement

This work received financial support from Na- tional Science Foundation Grant CHE83-16205, hereby gratefully acknowledged. The authors ap- preciate the valuable suggestions and advice from S.R. Gandhi, who also participated in some of the preliminary experimentation.

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