J . Phys. Chem. 1985,89, 3426-3429
3426
Change in the Mechanbm of Laser Multiphoton Ionization-Dissociation in Benzaldehyde by Changlng the Laser Pulse Width J. J. Yang,t D. A. Gobeli,t and M. A. El-Sayed* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024 (Received: November 13, 1984; In Final Form: March 12. 1985)
The effect of the laser pulse width on the multiphoton ionizationdissociation (MPID) of benzaldehyde is investigated. This is carried out by comparing the MPID mass spectra obtained at excitation wavelengths of 266 and 355 nm by a picosecond pulsed laser with those obtained previously with a nanosecond pulsed laser. The results suggest that the relative importance of the ladder and ladder-switching mechanisms changes with the laser pulse width. The role of ladder switching decreases as the laser pulse width decreases.
Introduction Two dominant mechanisms are used to explain laser multiphoton ionization-dissociation (MPID) of polyatomic molecules. These are the ladder mechanism and the ladder-switching mechanism. In the ladder mechanism, photon absorption occurs in the parent species from which all smaller fragments originate either directly or indirectly, i.e. p &!-
p+
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n3hv
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3 D
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- ...
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5 4' In the ladder-switching mechanism, in addition to the parent ions, photon absorption may also occur in the fragment ions,2 i.e. n h.
p"'p*
&p+
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npnr
n hv
(P+)*
J
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For ladder switching to occur, fragmentation of the parent ions must occur within the laser pulse width. Therefore, it is expected that the ladder mechanism should dominate at extremely short laser pulse widths. As the laser pulse width increases,contribution from the ladder-switching mechanism will increase. Benzaldehyde was selected for this study because detailed power-dependence MPID studies were performed previously with nanosecond pulsed lasers a t several excitation wavelength^.^ Indications from these studies are that ladder switching can occur t Resent address: Bell Laboratories, 1A-207, 600 Mountain Avenue, Murray Hill, NJ 07974. $Resent address: Northrop Corporation, Electronics Division, Hawthome, CA 90250.
0022-3654/85/2089-3426$01.50/0
in the parent molecule upon excitation at 266 nm and several nearby shorter wavelengths. Under these experimental conditions, it is impossible to observe the parent or parentlike ions alone even a t the lowest laser power used. This tpgether with the power dependence behavior in the low-power region suggested that excitation at 266 nm results in dissociation of benzaldehyde, excited into the S2vibronic manifold, into benzene and CO. The resulting benzene then absorbs further photons and ionizes (the DI channel, dissociation then ionization). The rate of dissocation is estimated to be greater than 1O1O s-l." Our n a n e n d results are consistent with the observation of Long et al.s in which the photoelectrons produced in the low-power MPID of benzaldehyde at 258.9 nm have a kinetic energy distribution that resembles the one for benzene. Their observations indicate that substantial benzene formation occurs within the 2 4 s laser pulse while exciting in the 258-259-nm region. If the dissociation time of this channel is actually in the range of 0.1-1.0 ns, we would not expect to observe ladder switching from the excited parent molecule using picosecond pulses. The purpose of the current research is to investigate the laser pulse width effect upon the MPID mechanisms. In this communicat&n, we present MPID power-dependence experiments of benzaldehyde using a picosecond pulsed laser at excitation wavelengths of 266 and 355 nm. The results are compared with their corresponding nanosecond result^.^
Experimental Section The experimental setup with the nanosecond pulsed laser has
been described previously.' The experimental apparatus for that study consisted of a nanosecond pulsed laser (Quanta-Ray DCR Nd:YAG), nanosecond pumped dye laser (Quanta-Ray PDL-l), and a time-of-flight mass spectrometer constructed in our labor atory . The picosecond wavelength and power dependence studies were performed using a pssively modelocked Nd:YAG laser (Quantel 470). The pulse widths of the third and fourth harmonics of the YAG output (355 and 266 nm) were approximately 25 ps. Power fluctuations were approximately *20%. The ions were produced inside the ionization region of another time-of-flight mass spectrometer whose ion optics consisted of a two-stage acceleration region similar to that described by Wiley and McLaren6 and beam (1) A. Gedankcn, M. B. Robin, and N. A. Kuebler, J . Phys. Chem., 86, 4096 (1982). (2) W. Dietz, H. J. Neusser, U. Boesl, E.W. Schlag, and S.H. Lm,Chem. Phys., 66, 105 (1982). (3) J. J. Yang, D. A. Gobeli, R. S.Pandolfi, and M. A. El-Sayed, J. Phys. Chem., 87, 2255 (1983). (4) V. M.Matyuk, A. V. Polevoi, V. K. Potapov, and A. L. Prokhoda, Hiah Enerm Chem. ( E n d . Tran.4.). 16, 7 5 (1982) (Kim.Vys. Energ., 16, 997 1982)F( 5 ) S.R. Long, J. T. Meek, P. J. Harrington, and M. P. Reilly, J . Chem. Phys., 78, 3341 (1983). (6) W. C. Wiley and I. H. McLaren, Rev.Sci. Instrum., 26, 1150 (1955).
0 1985 American Chemical Society
Multiphoton Ionization-Dissociation in Benzaldehyde centering plates for deflecting the ion beam along the time-of-flight axis. The field-free region of the mass spectrometer was approximately 2 m long. Ions were detected by a channel electron multiplier (Galileo Electro-Optics 48 16). The output of the channeltron was directed to a XlOO amplifier (Pacific Precision Instruments 2A44) and 'recorded on a transient digitizer (Biomation 8100). A number of digitized traces were then added with a ditial signal analyzer (Tracor Northern NS-570A). For maximum resolution, the transient digitizer was set at 16 ns/channel, which provided an interval of approximately 20 ws in which to record a mass spectrum. As a result, the H+ ion signal was frequently not in the recorded mass spehrum. The accumulated output spectrum from the digital signal analyzer was sent to an on-line computer (VAX/VMS 11/780) for analysis and plotting. A plano-convex quartz lens of 175-mm focal'length was used to focus the laser beam into the ionization region of the mass spectrometer. In the power-dependence experiments, the laser power was adjusted by using glass filters of known transmittances at the wavelengths used. The benzaldehyde wag obtained commercially (Mallinckrodt AR) and used without further purification. The sample pressure in the ionization region was kept low enough to assure that no interference from ion-molecule reactions occurred. The background pressure in the detection region was torr. The channeltron voltage was set at less than 5 X approximately -2800 V. The peak powers from the picosecond laser were, in general, higher than those from the nanosecond laser. The beam shape of the picosecond pulsed laser is Gaussian while that from the nanosecond pulsed laser is doughnut-shaped. The better beam quality of the picosecond laser provides better focusing conditions (Le., can be focused down more tightly).
Results and Discussion Low Laser Power Results. Figure 1 shows the MPID patterns of benzaldehyde obtained by using picosecond pulses at very low average laser powers (0.138 and 0.270 mW, respectively) at excitation wavelengths of 355 and 266 nm, respectively. Similar to the nanosecond MPID results at 355 nm (Figure 2),3 it is possible to almost exclusively generate parentlike ions with both excitation pulses. Unlike the nanosecond MPID results at 266 nm (Figure 2),3 it is possible using the picosecond pulses to exclyively generate the C7H50+ion under the lowest average power. This implies the absence of the DI channel under this latter experimental condition. Wit$ nanosecond excitation, the C7H50+ ion is considered to be produced via the parent ion7 With picosecond excitation, this pathway for the production of the C7H50+ ion should also be favored since the photon absorption rate must be on the picosecond time scale in order to produce these ions. The reason that the C7H50+ion is observed instead of the parent ion, C7H60+,at extremely low average power is probably due to the immediate dissociation of the parent ion into the C7H@+ ion and neutral hydrogen atom. This results from the large amount of excess energy contained in the parent ion after three-photon ionization with 266-nm radiation. Since the C7H50+ion is the only ion produced underthe experimental conditions mentioned above, this might imply that the appearance energies (measured from the energy of the benzaldehyde molecule which is set at zero) for other ions produced from the parent ion are higher than the three-photon energy a t 66 nm (14.01 eV). The fact that we do not observe the DI channel with picosecond excitation at 266 nm even at the lowest average power might be because dissociation occurs on a time scale longer than the picosecond laser pulse width. With the nanosecond excitation at 266 nm, the DI channel can be observed. If dissociation occurs within the picosecond pulse width, because of the higher pulse intensity of the picosecond excitation, the resulting benzene could be ionized, at least partially, and the DI channel could be observed. This should be the case unless the absorption cross sections for benzene in the picosecond excitation are much smaller (Le., approach zero) than those in the nanosecond excitation due to different extents of energy redistribution in the hot benzene
The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 3427 n
I
I
I-
H
Z
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3
I
355 In C,H,O*
h a
I
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M
z 0
20
40
eo
60
100
120
MASS NUMBER Figure 1. Picosecond MPID mass spectrum of benzaldehyde at X, = 355 nm (laser average power 0.138 mW) and A, = 266 nm (laser average power 0.270mW), respectively, in the low average power region.
X- 356 nm 'gHxt
c7iI .oi
h=i!66nm
I I + Figure 2. Nanosecond MPID mass spectrum of benzaldehyde at A,, = 355 nm (laser average power 0.42mW) and X, = 266 nm (laser average power 0.038 mW), respectively, in the low average power region.
molecule prior to photon absorptions. This is because multiphoton ionization of molecules can occur via nonresonant absorption, particularly under conditions of high peak intensities. Although the extreme case of zero absorption cross section cannot be excluded completely, it is very unlikely. Thus, it seems to be more reasonable that dissociation occurring on a time scale longer than the laser pulse width accounts for the observed picosecond result. This together with our nanosecond results sets the dissociation time a t an excess energy corresponding to one 266-nm photon above the ground state of the molecule to be between 8 ns and 25 ps, the pulse widths of the two lasers used. This range of dissociation time scale is roughly consistent with those estimated by both Matyuk et al.4 and Long et al.5 The time scale of energy redistribution processes in the excited parent molecules might be the rate-determining step leading to dissociation of the excited parent molecules. With nanosecond excitation, more complete energy redistribution can occur which would permit the DI channel to be observed. Since dissociation occurs on a time scale longer than the picosecond pulse width, it is unlikely that the DI channel will play a role in the observed mass spectrum with the picosecond excitation. The ladder mechanism in the parent molecule manifold is the dominant mechanism for producing the observed mass spectrum. This ladder mechanism increases its importance as the laser pulse width decreases. Low to Moderate Laser Power Results. Figure 3 show the MPID mass spectra of benzaldehyde produced by the picosecond excitation at 355 nm with several laser powers. As one increases the laser power in the low to moderate average power region, a larger C4H3+than C6H5+ion yield can be obtained when the parent ion yield is still relatively high. At the same excitation wavelength with nanosecond pulses, the C6H5+ion yield remains
Yang et al.
3428 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 Benzaldehyde
,,A
355 n m
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,
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40
60
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MASS NUMBER
Figure 3. Picosecond MPID mass spectrum of benzaldehyde at X, = 355 nm in the low to medium laser average power region.
higher than the C4H3+ion yield when the parent ion yield is high (Figure 4).3 This might suggest the direct production of C4H3+ from C7H60+is more important with picosecond than with nanosecond excitation (the average power range for each case is specified in Figure 4). On the other hand, the production of C4H3+ via c6Hx+ = 5 , 6 ) seems to be more important with nanosecond than with picosecond excitation. Here, the terminology "low to moderate laser power" is used for both the nanosecond and picosecond cases but does not imply the same peak laser powers in both instances. It only specifies the approximate power range where ions start to be observed to the power range where moderate fragmentation occurs. To interpret the different power-dependence behavior with picosecond and nanosecond excitation at 355 nm, let us first consider the following important competing pathways from the parent ion to form C4H3+:7
(x
\ v (4.65 a V )
(3e)
Based upon the heats of formation of each species concerned reaction channels i, ii, and iii are lower in energy relative to channels iv and v. The actual energy requirements for these reactions are unknown due to the unknown reverse activation energies. However, according to the threshold appearance energies deduced from the results with the picosecond pulses at 266 nm, reaction channels ii-v might require more 355-nm photons than channel i. It is very likely that reaction channel v might still be the higher energy channel among these important competing channels after taking into consideration the reverse activation energy. An increase in the excess internal energy of the parent ion before its fragmentation will increase the probability of the occurrence of higher energy channels around their threshold energy region.
We assume that, for the same excitation wavelength, the cross sections for photon absorptions from excited states of the benzaldehyde molecular ion to its higher energy states involved in the picosecond excitation are not smaller than those involved in the nanosecond excitation. The average power ranges used for comparison of power-dependence behavior (see Figures 3 and 4) are such that the peak intensities are higher in the picosecond than in the nanosecond experiments. This together with the photon absorption cross sections involved in the nanosecond and picosecond excitations results in larger absorption rates with picosecond excitation. At extremely low laser powers, the total number of photons absorbed per parent species can be limited to produce only the parent ion in both excitation cases. This is because both the rate of photon absorption and the pulse width will determine the total number of photons absorbed. An increase in laser peak intensity will lead to an increase in the mean excess energy deposited in each molecular system and thus may open new fragmentation channels. The increase in peak intensity also leads to an increase in the photon absorption rate as well as the number of photons absorbed (Le., excess internal energy) in the reactant ion before its fragmentation. The fragmentation rate, in general, increases rapidly with increasing excess internal energy imparted to the reactant ion. With nanosecond excitation, due to the smaller rate of photon absorption, the parent ion may start to fragment at lower excess energy. The dissociation does not necessarily occur from the dissociative state! Fragmentation to form less energy expensive ions such as C7H50+or C,H,+ (X= 5 , 6 ) is more likely to occur at lower excess energy. Further photon absorption could occur from these fragment ions if they are formed within the laser pulse width. With picosecond excitation, the parent ion will not enter the same fragmentation channel at the same amount of excess energy due to the higher rate of photon absorption provided with the picosecond laser pulses. More excess energy can be deposited into the parent ion before its fragmentation. Thus, fragmentation to form more energy expensive ions such as some of the C4-containing ions directly from parent ions will have a higher probability compared to that with the nanosecond excitation. Thus, in the low to moderate power region, more C4H3+ can be produced via C6H5+by the excitation ladder switching from parent ion manifold to C6Hx+ ion manifold with the nanosecond than with the picosecond excitation. With the picosecond excitation, more C4H3+can be produced directly from the parent ion. This means that the ladder switching from the parent ion manifold is at least partially eliminated upon the picosecond excitation in the low to moderate power region. It is also possible that, with the nanosecond excitation, more extensive energy redistribution can occur before further photon absorption occurs. This provides the possibility of fragmentation to form less energy expensive ions. With picosecond excitation, energy redistribution is less extensive before further photon absorption. Thus, further photon absorption wins in this case and fragmentation to form less energy expensive ions could be bypassed. More excess energy will be deposited in the parent ion before it starts to fragment upon picosecond excitation. This also demonstrates that ladder switching decreases its importance with decreasing laser pulse width.
Conclusion The mechanisms for producing MPID of benzaldehyde are found to be dependent on the laser pulse width. These mechanisms are summarized as follows: ( A ) X,,=266 nm, low laser power
(7) J. J. Yang, Ph.D. Dissertation, University of California at Los Angeles, 1984. (8) J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron, K. Draxl,
and F. H. Field, Narl. Stand. ReJ Data Ser. (U.S. Natl. Bur. Stand.).
NSRDS-NBS, 26 ( 1969). (9) H. M. Rqsenstock, K. Draxl, B. W. Steiner, and J. T. Herron,J . Phys. Chem. Ref. Data, 6, Suppl. 1 (1977). (10) R. G. McLoughlin and J. C. Traeger, Org. Mass Spectrom., 14,434 (1979). (11) T. Baer, G. D. Willett, D. Smith, and J. S. Philips, J . Chem. Phys., 70, 4076 (1979). (12) M. W. Chase, J. L. Curnutt, A. T. Hu, H. Prophet, A. N. Syverud and L. C. Walker, J. Phys. Chem. Ref Data, Suppl., 3, 311 (1974).
(ns excitation)
C,H,CHO*
C6H&O+. C,Hxt C6H5CHO+ (+nnr) D (ns. ps excitation)
(X-5-61
J. Phys. Chem. 1985,89, 3429-3434
3429
A- 3% nm
I
D
lv
L
i
0
I
I
probability increase with ps excitation
The ladder switching from the parent to benzene molecule manifold with 266-nm picosecond excitation is not observed even at the threshold power of ion observation. The ladder mechanism in the parent molecule manifold dominates for producing the observed MPID mass spectrum with 266-nm picosecond excitation. In addition, a t 355 nm the ladder-switching mechanism in the parent ion manifold is not as important with picosecond excitation compared to nanosecond excitation in the low to moderate power range.
am M
1
a42 M
i .
Figure 4. Nanosecond MPID mass spectrum of benzaldehyde at 355 nm in the low to medium laser average power region.
=
Acknowledgment. We acknowledge the financial support of the National Science Foundation. Registry No. Benzaldehyde,
100-52-7.
Theoretical Analysis of the Effects of Light Intensity on the Photocorrosion of Semiconductor Electrodes R. M. Bedto*+and A. J. Nozik* Solar Energy Research Institute, Golden, Colorado 80401 (Received: December 5, 1984)
A kinetic model was developed to describe the effects of light intensity on the photocorrosion of n-type semiconductorelectrodes. The model is an extension of previous work by Gomes and co-workers that includes the possibility of multiple steps for the oxidation reaction of the reducing agent in the electrolyte. Six cases are considered where the semiconductor decomposition reaction is multistep (each step involves a hole); the oxidation reaction of the reducing agent is multistep (each step after the first involves a hole or a chemical intermediate), and the first steps of the competing oxidation reactions are reversible or irreversible. It was found, contrary to previous results, that the photostability of semiconductor electrodes could increase with increased light intensity if the desired oxidation reaction of the reducing agent in the electrolyte was multistep with the first step being reversible.
Introduction Photocorrosion of semiconductor electrodes in photoelectrochemical cells is the major problem limiting the practical utilization of such cells for solar energy conversion. The problem is that photogenerated minority carriers (holes in n-type anodes and electrons in ptype cathodes) are powerful oxidizing and reducing agents, respectively, and generally are capable of oxidizing or reducing the semiconductor itself, in addition to the redox species in solution. Thus, there is a kinetic competition for the photogenerated minority carriers between the desired redox reaction in solution and the undesired redox reaction of the semiconductor electrode. In order to have stable semiconductor photoelectrodes, it is necessary to suppress the redox chemistry of the photoelectrode and to enhance that of the electrolyte. For electrochemical photovoltaic cells, where the electrolyte redox reactions only provide a route for charge transport, this is thesent Address: Departamentode Fisica, E.T.S.I. de Telammunicacion, Universidad Politecnica de Madrid, 28040 Madrid, Spain.
achieved by placing a fast, one-electron redox oouple in the solution that has much more favorable charge-transfer kinetics compared to the semiconductor decomposition reactions. Thus, for example, polysulfide ion (S>-)in 1 N N a O H electrolyte is preferentially oxidized by photogenerated holes in an n-CdS anode compared to the oxidation of the n-CdS to free sulfur and Cd2+.l” It is also generally observed that, for n-type systems, the more thermodynamically favored reactions are also kinetically favored. For photoelectrosynthetic cells, where fuels or chemicals are produced in the electrolyte via generally more complicated multistep redox reactions, the above described solution to the (1) Ellis, A. B.; Kaiser, S.W.; Wrighton, M. S.J. Am. Chem. Soc. 1976, 98, 1635. ( 2 ) Ellis, A. B.; Kaiser, S.W.; Wrighton, M. S.J. Am. Chem. Soc. 1976, 98, 6855. (3) Hodes, G.; Manassen, J.; Cohcn, D. Nature (London) 1976,261,403. (4) Miller, B.; Hellcr, A. Noture (London) 1976, 262, 680. ( 5 ) Heller, A.; Chang, K.C.; Miller, B. J . Electrochem. Soc. 1977, 124, 697.
0022-365418512089-3429%01SO10 0 1985 American Chemical Society
