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Photodissociation of alkyl nitrites adsorbed on an MgF2 surface. Rotational and translational energy distributions of product NO(ν, J) molecules
6 December 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 263 (1996) 19-24
Photodissociation of alkyl nitrites adsorbed on an MgFe surface. Rotational and translational energy distributions of product NO(v, J) molecules C . J . S . M . S i m p s o n a, RT. G r i f f i t h s a, H . L . W a l l a a r t a, M. T o w r i e h a Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OXI 3QZ, UK b Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OXll OQX, UK Received 9 August 1996; in final form 28 August 1996
Abstract The 351 nm photolysis of tert- and iso-butyl nitrite adsorbed on MgF2 held at 75 K has been investigated using resonanceenhanced multi-photon ionisation spectroscopy. Low desorption laser fluences were used to preclude thermal effects. The rotational distribution of desorbed NO(v" = 2) is velocity-dependent, being principally Gaussian at 2000 m s -~ and Boltzmann below 600 m s -~. The fast molecules suffer no collisions before desorption and their energy distributions are similar to those found in the gas phase. The slower molecules have suffered collisions. At intermediate velocities, the observed distribution is the sum of these two components which populate high and low rotational states respectively.
1. Introduction The photolysis of alkyl nitrites, RONO, in the gas phase has been the subject of many experimental and theoretical studies [ 1-8]. In order to understand the dynamics of photochemical processes occurring on dielectric surfaces, we consider that it is most fruitful to compare processes in the gas phase with those occurring on the surface. By comparing the (v, J) state of the desorbed products with gas phase data, the role of the surface and surrounding molecules can be inferred. The gas phase absorption spectrum of the alkyl nitrites is structured in the 300-400 nm region. This corresponds to the A IA" ,--- X lAr (rr* ~-- n) transition and excitation of the N=O stretching mode of the excited state. Once excited to the A 1A" state, dissociation follows within 300 fs to give an alkoxy fragment, RO, and vibrationally excited NO [6]. The vibrational
distribution of the product NO is found to depend on the vibrational excitation in the parent molecule, peaking one quantum below the initial vibrational excitation [6,9-11 ]. Our initial experiments on the photolysis of adsorbed nitrites have shown that the vibrational distribution of the desorbed NO is the same as that found in the gas phase at the same wavelength [ 12]. Having established that vibrational deactivation of NO is not significant on the timescale of desorption, we have turned our attention to the rotational distribution of the desorbed molecules. In our study of the dissociation of adsorbed 3-cyclopentenone [ 13 ], the rotational temperature of the CO product was found to increase with increasing velocity. We have also shown that NO produced from the photolysis of adsorbed NO dimers has a Gaussian distribution of rotational states at the highest velocities, whilst at lower velocities a Boltzmann distribution with Trot = 300-4-50 K is found
0009-2614/96/$12.00 Copyright (~) 1996 Elsevier Science B.V. All rights reserved. PH S 0 0 0 9 - 2 6 14 ( 9 6 ) 001 1 80-3
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CJ.S.M. Simpson et aL/Chemical Physics Letters 263 (1996) 19-24
[ 14]. In that instance, there were no gas phase data taken at 248 nm with which to compare our results. The dissociation of alkyl nitrites in the gas phase produces NO with an inverted rotational distribution characterized by a Gaussian form. In the case of tertbutyl nitrite produced in (v" = 1), this distribution is centred on J = 28.5 [5]. It is hoped that knowledge of the rotational distribution of the NO product in the gas phase will enable us to gain a more detailed insight into the relaxation processes occurring on the surface.
2. Experimental A detailed description of the ultra-high vacuum chamber is given in Ref. [ 13]. Only a brief account will be given here. The chamber was diffusionpumped to ~< 2 × 10 - j ° Torr, with additional cryopumping by liquid-nitrogen and liquid-helium cooled shrouds within the chamber. The single crystal MgF2 surface, 18 mm in diameter and 0.5 mm thick, was cooled with liquid helium. The temperature of the top face of the crystal was measured with a silicon diode. A small heater circuit, in conjunction with a feedback system could hold the surface between 30 and 300 K to within ± l K. For this work, the surface was held at 75 + 1 K. This is above the desorption temperature of NO from MgF2, and ensured that all NO produced from the photolysis of molecules in the top layer was desorbed. Molecules were dosed onto the surface using a microchannel array fed by a 2/xm pinhole. This provided an evenly dosed area of 5 mm diameter on the crystal face. Measurements taken with our mass spectrometer showed that all nitrite molecules stuck to the surface during dosing. A description of the doser calibration is given in Ref. [ 14]. The layer thickness was estimated to be between two and four monolayers. Samples of iso- and tert-butyl nitrite (of 95% and 90% purity respectively) were obtained from Aldrich and were used following at least three freeze-pumpthaw cycles. The output of an excimer laser, operating at 351 nm, was used to photolyse the adsorbed molecules. The central portion of the beam, which is of even fluence, was used to illuminate a mask and was then imaged onto the surface to coincide with the 5 mm diameter dosed spot. Desorption laser fluences of ~<1 mJ cm -2
were used throughout. Detection of the desorbed NO(v, J) molecules was by ( 1÷1 ) REMPI via the A 23~+ state as in Ref. [ 14]. The frequency-doubled output of an excimer-pumped dye laser system was used to ionize the NO, and the ions thus formed were deflected onto a Galileo microchannel plate by a series of charged surfaces based on the Wiley-McClaren design [ 15]. The output of the dye laser was around 2 mJ per pulse, and the beam was of approximately 4 mm diameter in the interaction region. In this case, the first transition was saturated and the population of each rovibrational level of the ground state was directly proportional to the ion current at the transition frequency. The delay between the desorption and detection lasers was controlled by a digital delay unit and a repetition rate of 5 Hz was used throughout.
3. Results REMPI spectra of the (0, 2) y-band of NO produced from the photolysis of adsorbed iso- and tertbutyl nitrite were taken. The dependence of the rotational distribution on the velocity of the desorbed NO was investigated, and time of flight (TOF) distributions of selected rotational levels were also measured by varying the delay between photolysis and detection lasers. 3.1. Translational energy distribution
TOF distributions were measured for the rotational levels, J = 9.5 (R22 branch), 20.5 (P22 + Ql2 branch) and 35.5 (Qll + P2J branch) oftert-butyl nitrite. Fig. 1 shows that the highest energy rotational levels are only found over a narrow range of velocities. By integrating the distributions we estimate that the population of the J = 35.5 level is 1.5 -4- 0.5 times that of the J = 9.5 level. 3.2. Rotational energy distributions
The effect of velocity over the range 1800 to 500 m s - 1 on the rotational distribution of the desorbed NO was investigated. Spectra were taken for velocities of 2000, 1800, 1600, 1400, 1200, 900, 650 and 500 m s -J . No product was desorbed with a velocity greater
C.J.S.M. Simpson et al./Chemical Physics Letters 263 (1996) 19-24
Velocity ms -1 2000 1000 30II~""l ....
3oo
500
200
I
(a)
21
than 2000 m s - t, and there was no population of rotational states above J = 48.5. Rotational distributions measured using the R11Q21 branch for iso-butyl nitrite are shown in Fig. 2 and for tert-butyl nitrite in Fig. 3. Above 1400 m s - I , the rotational distribution is well-characterized by a Gaussian function although there is some population of low-J rotational states. The function has the form
20
N( J) = A e x p [ - l n 2 × ( J - Jmax)2/( AJ)2]. 15 10
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Flight time / ItS Fig. I. The variation in TOF distribution of NO desorbed in (t ,/r = 2) with rotational level. (a) J = 35.5 (Qtl + P21 branch). (b) J = 20.5 (P22 + QI2 branch). (c) J = 9.5 (R22 branch).
and in the case of iso-butyl nitrite Jmax = 30.5 + 1 and AJ = 8. For tert-butyl nitrite, Jmax = 26.5 ± 1 and A J = 4. The results are shown in Figs. 2a and 3a. We have shown that NO desorbed with a velocity of 500 m s-m has a rotational distribution characterized by a Boltzmann function with Trot = 400 ~ 100 K [ 12 ]. This description of the rotational distribution by a Boltzmann function is arbitrary if the process departs from thermodynamic equilibrium but it is a useful guide in describing our results. By analysing the rotational distribution as a function of velocity, we discovered how the change from a Gaussian rotational distribution to Boltzmann occurs. In the case of iso-butyl nitrite, the rotational distribution of the desorbed NO changes from one to the other via a distribution which is the sum of Boltzmann and Gaussian functions. The population of states below J = 20.5 increases with decreasing velocity and may be described by a function with a characteristic temperature of 500 ± 200 K over the range 500 to 1400 m s - l . The data in Figs. 2b, 2c and 2d were fitted by the sum of Boltzmann and Gaussian functions with Trot = 500 ~z 200 K, with the temperature of the Boltzmann part allowed to float within the fitting procedure in Figs. 2c and 2d while in Fig. 2b, the rotational temperature was held constant at 500 K. Fig. 2e shows a Boltzmann distribution with Trot = 390 + 50 K. The rotational distribution of NO desorbed from tert-butyl nitrite shows a similar behaviour. However, because the distribution is centred on J = 26.5, which significantly populates states below J = 15.5, it is not possible to reliably fit the measured populations as a sum of two functions as above. A similar trend in behaviour is seen with population of states below J = 15.5 increasing with decreasing velocity, as shown in Figs. 3b-3d in which the data are fitted by a sum of Boltzmann and Gaussian functions with the tempera-
C.J.S.M. Simpson et al./Chemical Physics Lettem 263 (1996) 19-24
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Fig. 2. The rotational distribution o f N O ( v ~ = 2) desorbed from iso-butyl nitrite, measured as a function o f velocity. ( a ) 1800 m s - l . ( b ) 1 4 0 0 m s - ] . ( c ) l l 0 0 m s - l . ( d ) 9 0 0 m s - j . ( e ) 500 "
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Rotational quantum number, J Fig. 3. The rotational distribution of N O ( v " = 2) desorbed from ten-butyl nitrite, measured as a function o f velocity. ( a ) 2000 m s - i . ( b ) 1500 m s - t . ( c ) 1400 m s - 1 . ( d ) 1200 m s - l . ( e ) 650 m s - l .
C.J.S.M. Simpson et al./Chemical Physics Letters 263 (1996) 19-24
ture of the Boltzmann part held at 700 K. Fig. 3e shows a Boitzmann distribution with Trot = 370 + 40 K.
3.3. Coverage experiments Approximately one monolayer of krypton was dosed on top of the nitrite surface. This had the effect of quenching the signal from the N O ( v " = 2) level, and shows 'that desorption from the topmost layer is the dominant process at the surface.
4. Discussion Our previous experiments on the photolysis of tertand iso-butyl nitrite established that the vibrational distribution of the desorbed NO is the same as that found under collision-free conditions in the gas phase to within experimental error [ 12]. It was concluded that, under the conditions of low desorption laser fluence used, the dynamics of the dissociation are not perturbed by the presence of the surrounding molecules or the MgF2 surface and that the desorbing NO molecule does not suffer enough collisions to cause vibrational relaxation. The rotational energy distribution of the product NO is a more sensitive probe of the processes occurring on the surface since it is easily distorted by collisions. The rotational distributions have been measured as a function of velocity to establish what, if any, correlations exist between these two degrees of freedom in the desorbed NO. At the highest velocities, a Gaussian distribution of rotational states is found with the centre and width of the Gaussian function varying with the parent molecule. These distributions closely resemble those seen in the gas phase [4,5]. The degree of rotational and translational excitation in the desorbed NO is also high: at a velocity of 2000 m s - I , corresponding to a kinetic energy of 5000 cm -1 , the rotational energy is 1200 c m - l, of the same order as gas phase results. One possible origin of these highly excited molecules might be the thermal desorption of intact alkyl nitrites, followed by photolysis in the gas phase, during the same laser pulse. Our results from the photolysis of (NO)2 at 248 nm also showed a Gaussian distribution of rotational states at the highest velocities [14]. In this case, all NO is produced on the
23
surface since NO dimers do not absorb significantly in the gas phase at this wavelength. Unlike (NO)E, alkyl nitrites absorb at 351 nm to give NO with a Gaussian distribution of rotational states. Thus, photolysis of intact desorbed nitrites in the gas phase could contribute to the observed yield of fast molecules. Since these fast molecules constitute about half the total yield of desorbed NO, a two-photon process such as this would have to be very efficient. In subsequent experiments [ 16], ( 1 + 1') REMPI detection of NO has been used to detect both fast and slow components of the desorbed NO. This scheme is about 20 times more sensitive than a ( l + l ) scheme [ 17] and allowed us to work with a desorption laser fluence of 0.05 mJ cm -2 - 20 times lower than the 1 mJ cm -2 used here. If the fast molecules were the result of a two-photon process, there would be a dramatic decrease of around 400 times in the intensity of these fast molecules. No such decrease was seen, indicating that, as in the case of (NO)2, the fast molecules with a Gaussian distribution of rotational states are the result of a one-photon process. We conclude that NO is born on the surface highly rotationally excited and that some is desorbed having suffered no collisional relaxation of the rotational and translational energies. These results show a number of similarities with the work of Kades et. al. [5]. In their study of the dissociation of alkyl nitrite clusters in the gas phase they found a fast component of the product NO with a Gaussian rotational distribution, which was produced in the dissociation of nitrite monomers, and a slow component with a low degree of rotational excitation, having around 100 cm -I of rotational energy and almost no translational kinetic energy, which was the result of collisional relaxation of the NO within the cluster. There was also significant vibrational relaxation of the NO produced from the photolysis of ten-butyl nitrite clusters. In our experiments, no vibrational relaxation of the desorbed NO was observed. The desorbed NO is also much more rotationally and translationally excited and so we conclude that there is no significant cluster formation within the adsorbate layer. In previous studies, NO desorbed with a velocity of 500 m s -1 was shown to have a rotational temperature of 400 ± 100 K [ 12]. It is known that very few collisions between NO and surrounding molecules are required to deactivate the rotational and translational energies with many more required for vibrational de-
24
C.J.S.M. Simpson et al./Chemical Physics Letters 263 (1996) 19-24
activation. The high efficiency o f rotational relaxation results in complete transfer o f population from high to low rotational states prior to desorption with no significant population of intermediate states. Collisions prior to desorption also establish a rotational distribution which has a Boltzmann form, peaking around J = 8.5, and significantly reduce the amount of translational energy. Coverage experiments have shown that desorption from the top layer is the dominant process in this system and it is therefore unlikely that this second form o f NO is desorbed from deep within the adsorbate layer. By probing the rotational distribution o f the desorbed NO as a function o f velocity, we have shown that NO is produced in two forms: the first form is ejected directly into the gas phase and is highly excited, characterized by a Gaussian distribution of rotational states and high translational velocities. It is observed over the range o f velocities from 900 to 2000 m s - l . The second form, which results from collisions between the desorbing NO and surrounding molecules, has significantly less rotational and translational energies and is found over a wider range o f velocities. Only the relative proportion o f these two forms is found to change with velocity. The orientation o f the O - N = O moiety relative to the surrounding molecules may control the state of the desorbed product. The O - N = O group may point away from the surface and the surrounding molecules. In this case the product NO will be ejected directly into the gas phase. Alternatively, the ONO group may lie flat on the surface and any product NO will suffer a number o f collisions prior to desorption. In future experiments, we shall use infrared and photofragment yield spectroscopy to study the structure o f the adsorbate layer.
5. Conclusion The rotational distribution of N O ( v " = 2) desorbed from the photolysis o f iso- and tert-butyl nitrite has been shown to be depend on its velocity. The distribution changes from being principally Gaussian at the highest velocities to Boltzmann at the lowest. We consider that the Gaussian distributions arise from desorbed molecules which have suffered no col-
lisions. Those with Boltzmann distributions probably arise from sites which constrain them to suffer collisions before escaping from the surface.
Acknowledgement We wish to thank the staff of the workshops o f the PTCL for their support and those of the RAL, where these experiments were performed. In particular we wish to acknowledge the contribution o f W. Twigger. PTG thanks the PTCL for a maintenance grant.
References I IIH. Reisler, M. Noble and C. Wittig, in: Molecular photodissociation dynamics, eds. M.N.R. Ashfold and J.E. Baggot (Royal Society of Chemistry, London, 1987) p. 162. 121 U. Briihlmann, M. Dubs and J.R. Huber, J. Chem. Phys 86 (1987) 1249. 131 M.R.S. McCoustra, M. Hippler and J. Pfab, Chem. Phys. Lett. 200 (1992) 451. 141 D. Schwartz-Lavi and S. Rosenwaks, J. Chem. Phys. 88 ( 1988 ) 6922. [51 E. Kades, M. ROsslein, U. Bruhlmann and J.R. Huber, J. Phys. Chem 97 (1993) 989. 161 J.M. Mestdagh, M. Berdah, I. Dimcoli, M. Mons, P. Meynadier, P. d'Olieira, E Piuzzi, J.P. Visticot, C. Jouvet, C. Lardeux-Dedonder, S. Martrenchard-Barra, B. Soep and D. Solgadi J. Chem. Phys. 103 (1995) 1013. [71 M. Nonella, J.R. Huber, A. Untch and R. Schinke, J. Chem. Phys. 91 (1989) 194. 181 A. Untch, R. Schinke, R. Cotting and J.R. Huber, J. Chem. Phys 99(1993) 9553 . 191 O. Benoist d'Azy, E Lahmani, C. Lardeux and D. Solgadi, Chem. Phys. 94 (1984) 247. [ 101 S.A. Reid, J.T. Brandon and H. Reisler, Chem. Phys. Lett. 209 (1993) 22. [ I I I E. Kades, M. R6sslein and J.R. Huber, Chem. Phys. Lett. 209 (1993) 275. [ 121 C.J.S.M. Simpson, P.T. Griffiths and M. Towrie, Chem. Phys. Lett. 234 (1995) 203. [13] C.J.SM. Simpson, P.T. Griffiths, R.L. Lovegrove, P. Matousek and M. Towfie, Chem. Phys. Lett. 246 (1995) 269. [ 14] C.J.S.M. Simpson, P.T. Griffiths, J.M. Curry and M. Towrie, Chem. Phys. Lett. 250 (1996) 342. J 15] W.C. Riley and I.H. McClaren, Rev. Sci. Instrum. 26 (1955) 1150. 1161 C.J.S.M. Simpson, P.T. Gfiffiths, W. Twigger and M. Towrie, in preparation. [ 171 M. Hippler and J. Pfab, Chem. Phys. Lett. 243 (1995) 500.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 263 (1996) 19-24
Photodissociation of alkyl nitrites adsorbed on an MgFe surface. Rotational and translational energy distributions of product NO(v, J) molecules C . J . S . M . S i m p s o n a, RT. G r i f f i t h s a, H . L . W a l l a a r t a, M. T o w r i e h a Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OXI 3QZ, UK b Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OXll OQX, UK Received 9 August 1996; in final form 28 August 1996
Abstract The 351 nm photolysis of tert- and iso-butyl nitrite adsorbed on MgF2 held at 75 K has been investigated using resonanceenhanced multi-photon ionisation spectroscopy. Low desorption laser fluences were used to preclude thermal effects. The rotational distribution of desorbed NO(v" = 2) is velocity-dependent, being principally Gaussian at 2000 m s -~ and Boltzmann below 600 m s -~. The fast molecules suffer no collisions before desorption and their energy distributions are similar to those found in the gas phase. The slower molecules have suffered collisions. At intermediate velocities, the observed distribution is the sum of these two components which populate high and low rotational states respectively.
1. Introduction The photolysis of alkyl nitrites, RONO, in the gas phase has been the subject of many experimental and theoretical studies [ 1-8]. In order to understand the dynamics of photochemical processes occurring on dielectric surfaces, we consider that it is most fruitful to compare processes in the gas phase with those occurring on the surface. By comparing the (v, J) state of the desorbed products with gas phase data, the role of the surface and surrounding molecules can be inferred. The gas phase absorption spectrum of the alkyl nitrites is structured in the 300-400 nm region. This corresponds to the A IA" ,--- X lAr (rr* ~-- n) transition and excitation of the N=O stretching mode of the excited state. Once excited to the A 1A" state, dissociation follows within 300 fs to give an alkoxy fragment, RO, and vibrationally excited NO [6]. The vibrational
distribution of the product NO is found to depend on the vibrational excitation in the parent molecule, peaking one quantum below the initial vibrational excitation [6,9-11 ]. Our initial experiments on the photolysis of adsorbed nitrites have shown that the vibrational distribution of the desorbed NO is the same as that found in the gas phase at the same wavelength [ 12]. Having established that vibrational deactivation of NO is not significant on the timescale of desorption, we have turned our attention to the rotational distribution of the desorbed molecules. In our study of the dissociation of adsorbed 3-cyclopentenone [ 13 ], the rotational temperature of the CO product was found to increase with increasing velocity. We have also shown that NO produced from the photolysis of adsorbed NO dimers has a Gaussian distribution of rotational states at the highest velocities, whilst at lower velocities a Boltzmann distribution with Trot = 300-4-50 K is found
0009-2614/96/$12.00 Copyright (~) 1996 Elsevier Science B.V. All rights reserved. PH S 0 0 0 9 - 2 6 14 ( 9 6 ) 001 1 80-3
20
CJ.S.M. Simpson et aL/Chemical Physics Letters 263 (1996) 19-24
[ 14]. In that instance, there were no gas phase data taken at 248 nm with which to compare our results. The dissociation of alkyl nitrites in the gas phase produces NO with an inverted rotational distribution characterized by a Gaussian form. In the case of tertbutyl nitrite produced in (v" = 1), this distribution is centred on J = 28.5 [5]. It is hoped that knowledge of the rotational distribution of the NO product in the gas phase will enable us to gain a more detailed insight into the relaxation processes occurring on the surface.
2. Experimental A detailed description of the ultra-high vacuum chamber is given in Ref. [ 13]. Only a brief account will be given here. The chamber was diffusionpumped to ~< 2 × 10 - j ° Torr, with additional cryopumping by liquid-nitrogen and liquid-helium cooled shrouds within the chamber. The single crystal MgF2 surface, 18 mm in diameter and 0.5 mm thick, was cooled with liquid helium. The temperature of the top face of the crystal was measured with a silicon diode. A small heater circuit, in conjunction with a feedback system could hold the surface between 30 and 300 K to within ± l K. For this work, the surface was held at 75 + 1 K. This is above the desorption temperature of NO from MgF2, and ensured that all NO produced from the photolysis of molecules in the top layer was desorbed. Molecules were dosed onto the surface using a microchannel array fed by a 2/xm pinhole. This provided an evenly dosed area of 5 mm diameter on the crystal face. Measurements taken with our mass spectrometer showed that all nitrite molecules stuck to the surface during dosing. A description of the doser calibration is given in Ref. [ 14]. The layer thickness was estimated to be between two and four monolayers. Samples of iso- and tert-butyl nitrite (of 95% and 90% purity respectively) were obtained from Aldrich and were used following at least three freeze-pumpthaw cycles. The output of an excimer laser, operating at 351 nm, was used to photolyse the adsorbed molecules. The central portion of the beam, which is of even fluence, was used to illuminate a mask and was then imaged onto the surface to coincide with the 5 mm diameter dosed spot. Desorption laser fluences of ~<1 mJ cm -2
were used throughout. Detection of the desorbed NO(v, J) molecules was by ( 1÷1 ) REMPI via the A 23~+ state as in Ref. [ 14]. The frequency-doubled output of an excimer-pumped dye laser system was used to ionize the NO, and the ions thus formed were deflected onto a Galileo microchannel plate by a series of charged surfaces based on the Wiley-McClaren design [ 15]. The output of the dye laser was around 2 mJ per pulse, and the beam was of approximately 4 mm diameter in the interaction region. In this case, the first transition was saturated and the population of each rovibrational level of the ground state was directly proportional to the ion current at the transition frequency. The delay between the desorption and detection lasers was controlled by a digital delay unit and a repetition rate of 5 Hz was used throughout.
3. Results REMPI spectra of the (0, 2) y-band of NO produced from the photolysis of adsorbed iso- and tertbutyl nitrite were taken. The dependence of the rotational distribution on the velocity of the desorbed NO was investigated, and time of flight (TOF) distributions of selected rotational levels were also measured by varying the delay between photolysis and detection lasers. 3.1. Translational energy distribution
TOF distributions were measured for the rotational levels, J = 9.5 (R22 branch), 20.5 (P22 + Ql2 branch) and 35.5 (Qll + P2J branch) oftert-butyl nitrite. Fig. 1 shows that the highest energy rotational levels are only found over a narrow range of velocities. By integrating the distributions we estimate that the population of the J = 35.5 level is 1.5 -4- 0.5 times that of the J = 9.5 level. 3.2. Rotational energy distributions
The effect of velocity over the range 1800 to 500 m s - 1 on the rotational distribution of the desorbed NO was investigated. Spectra were taken for velocities of 2000, 1800, 1600, 1400, 1200, 900, 650 and 500 m s -J . No product was desorbed with a velocity greater
C.J.S.M. Simpson et al./Chemical Physics Letters 263 (1996) 19-24
Velocity ms -1 2000 1000 30II~""l ....
3oo
500
200
I
(a)
21
than 2000 m s - t, and there was no population of rotational states above J = 48.5. Rotational distributions measured using the R11Q21 branch for iso-butyl nitrite are shown in Fig. 2 and for tert-butyl nitrite in Fig. 3. Above 1400 m s - I , the rotational distribution is well-characterized by a Gaussian function although there is some population of low-J rotational states. The function has the form
20
N( J) = A e x p [ - l n 2 × ( J - Jmax)2/( AJ)2]. 15 10
L
0
"r"~
.
. . . . . . . .
I
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•
,
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i
i
,
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.
50
.
.
100
.
150
200
250
Flight time / ItS Fig. I. The variation in TOF distribution of NO desorbed in (t ,/r = 2) with rotational level. (a) J = 35.5 (Qtl + P21 branch). (b) J = 20.5 (P22 + QI2 branch). (c) J = 9.5 (R22 branch).
and in the case of iso-butyl nitrite Jmax = 30.5 + 1 and AJ = 8. For tert-butyl nitrite, Jmax = 26.5 ± 1 and A J = 4. The results are shown in Figs. 2a and 3a. We have shown that NO desorbed with a velocity of 500 m s-m has a rotational distribution characterized by a Boltzmann function with Trot = 400 ~ 100 K [ 12 ]. This description of the rotational distribution by a Boltzmann function is arbitrary if the process departs from thermodynamic equilibrium but it is a useful guide in describing our results. By analysing the rotational distribution as a function of velocity, we discovered how the change from a Gaussian rotational distribution to Boltzmann occurs. In the case of iso-butyl nitrite, the rotational distribution of the desorbed NO changes from one to the other via a distribution which is the sum of Boltzmann and Gaussian functions. The population of states below J = 20.5 increases with decreasing velocity and may be described by a function with a characteristic temperature of 500 ± 200 K over the range 500 to 1400 m s - l . The data in Figs. 2b, 2c and 2d were fitted by the sum of Boltzmann and Gaussian functions with Trot = 500 ~z 200 K, with the temperature of the Boltzmann part allowed to float within the fitting procedure in Figs. 2c and 2d while in Fig. 2b, the rotational temperature was held constant at 500 K. Fig. 2e shows a Boltzmann distribution with Trot = 390 + 50 K. The rotational distribution of NO desorbed from tert-butyl nitrite shows a similar behaviour. However, because the distribution is centred on J = 26.5, which significantly populates states below J = 15.5, it is not possible to reliably fit the measured populations as a sum of two functions as above. A similar trend in behaviour is seen with population of states below J = 15.5 increasing with decreasing velocity, as shown in Figs. 3b-3d in which the data are fitted by a sum of Boltzmann and Gaussian functions with the tempera-
C.J.S.M. Simpson et al./Chemical Physics Lettem 263 (1996) 19-24
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C.J.S.M. Simpson et al./Chemical Physics Letters 263 (1996) 19-24
ture of the Boltzmann part held at 700 K. Fig. 3e shows a Boitzmann distribution with Trot = 370 + 40 K.
3.3. Coverage experiments Approximately one monolayer of krypton was dosed on top of the nitrite surface. This had the effect of quenching the signal from the N O ( v " = 2) level, and shows 'that desorption from the topmost layer is the dominant process at the surface.
4. Discussion Our previous experiments on the photolysis of tertand iso-butyl nitrite established that the vibrational distribution of the desorbed NO is the same as that found under collision-free conditions in the gas phase to within experimental error [ 12]. It was concluded that, under the conditions of low desorption laser fluence used, the dynamics of the dissociation are not perturbed by the presence of the surrounding molecules or the MgF2 surface and that the desorbing NO molecule does not suffer enough collisions to cause vibrational relaxation. The rotational energy distribution of the product NO is a more sensitive probe of the processes occurring on the surface since it is easily distorted by collisions. The rotational distributions have been measured as a function of velocity to establish what, if any, correlations exist between these two degrees of freedom in the desorbed NO. At the highest velocities, a Gaussian distribution of rotational states is found with the centre and width of the Gaussian function varying with the parent molecule. These distributions closely resemble those seen in the gas phase [4,5]. The degree of rotational and translational excitation in the desorbed NO is also high: at a velocity of 2000 m s - I , corresponding to a kinetic energy of 5000 cm -1 , the rotational energy is 1200 c m - l, of the same order as gas phase results. One possible origin of these highly excited molecules might be the thermal desorption of intact alkyl nitrites, followed by photolysis in the gas phase, during the same laser pulse. Our results from the photolysis of (NO)2 at 248 nm also showed a Gaussian distribution of rotational states at the highest velocities [14]. In this case, all NO is produced on the
23
surface since NO dimers do not absorb significantly in the gas phase at this wavelength. Unlike (NO)E, alkyl nitrites absorb at 351 nm to give NO with a Gaussian distribution of rotational states. Thus, photolysis of intact desorbed nitrites in the gas phase could contribute to the observed yield of fast molecules. Since these fast molecules constitute about half the total yield of desorbed NO, a two-photon process such as this would have to be very efficient. In subsequent experiments [ 16], ( 1 + 1') REMPI detection of NO has been used to detect both fast and slow components of the desorbed NO. This scheme is about 20 times more sensitive than a ( l + l ) scheme [ 17] and allowed us to work with a desorption laser fluence of 0.05 mJ cm -2 - 20 times lower than the 1 mJ cm -2 used here. If the fast molecules were the result of a two-photon process, there would be a dramatic decrease of around 400 times in the intensity of these fast molecules. No such decrease was seen, indicating that, as in the case of (NO)2, the fast molecules with a Gaussian distribution of rotational states are the result of a one-photon process. We conclude that NO is born on the surface highly rotationally excited and that some is desorbed having suffered no collisional relaxation of the rotational and translational energies. These results show a number of similarities with the work of Kades et. al. [5]. In their study of the dissociation of alkyl nitrite clusters in the gas phase they found a fast component of the product NO with a Gaussian rotational distribution, which was produced in the dissociation of nitrite monomers, and a slow component with a low degree of rotational excitation, having around 100 cm -I of rotational energy and almost no translational kinetic energy, which was the result of collisional relaxation of the NO within the cluster. There was also significant vibrational relaxation of the NO produced from the photolysis of ten-butyl nitrite clusters. In our experiments, no vibrational relaxation of the desorbed NO was observed. The desorbed NO is also much more rotationally and translationally excited and so we conclude that there is no significant cluster formation within the adsorbate layer. In previous studies, NO desorbed with a velocity of 500 m s -1 was shown to have a rotational temperature of 400 ± 100 K [ 12]. It is known that very few collisions between NO and surrounding molecules are required to deactivate the rotational and translational energies with many more required for vibrational de-
24
C.J.S.M. Simpson et al./Chemical Physics Letters 263 (1996) 19-24
activation. The high efficiency o f rotational relaxation results in complete transfer o f population from high to low rotational states prior to desorption with no significant population of intermediate states. Collisions prior to desorption also establish a rotational distribution which has a Boltzmann form, peaking around J = 8.5, and significantly reduce the amount of translational energy. Coverage experiments have shown that desorption from the top layer is the dominant process in this system and it is therefore unlikely that this second form o f NO is desorbed from deep within the adsorbate layer. By probing the rotational distribution o f the desorbed NO as a function o f velocity, we have shown that NO is produced in two forms: the first form is ejected directly into the gas phase and is highly excited, characterized by a Gaussian distribution of rotational states and high translational velocities. It is observed over the range o f velocities from 900 to 2000 m s - l . The second form, which results from collisions between the desorbing NO and surrounding molecules, has significantly less rotational and translational energies and is found over a wider range o f velocities. Only the relative proportion o f these two forms is found to change with velocity. The orientation o f the O - N = O moiety relative to the surrounding molecules may control the state of the desorbed product. The O - N = O group may point away from the surface and the surrounding molecules. In this case the product NO will be ejected directly into the gas phase. Alternatively, the ONO group may lie flat on the surface and any product NO will suffer a number o f collisions prior to desorption. In future experiments, we shall use infrared and photofragment yield spectroscopy to study the structure o f the adsorbate layer.
5. Conclusion The rotational distribution of N O ( v " = 2) desorbed from the photolysis o f iso- and tert-butyl nitrite has been shown to be depend on its velocity. The distribution changes from being principally Gaussian at the highest velocities to Boltzmann at the lowest. We consider that the Gaussian distributions arise from desorbed molecules which have suffered no col-
lisions. Those with Boltzmann distributions probably arise from sites which constrain them to suffer collisions before escaping from the surface.
Acknowledgement We wish to thank the staff of the workshops o f the PTCL for their support and those of the RAL, where these experiments were performed. In particular we wish to acknowledge the contribution o f W. Twigger. PTG thanks the PTCL for a maintenance grant.
References I IIH. Reisler, M. Noble and C. Wittig, in: Molecular photodissociation dynamics, eds. M.N.R. Ashfold and J.E. Baggot (Royal Society of Chemistry, London, 1987) p. 162. 121 U. Briihlmann, M. Dubs and J.R. Huber, J. Chem. Phys 86 (1987) 1249. 131 M.R.S. McCoustra, M. Hippler and J. Pfab, Chem. Phys. Lett. 200 (1992) 451. 141 D. Schwartz-Lavi and S. Rosenwaks, J. Chem. Phys. 88 ( 1988 ) 6922. [51 E. Kades, M. ROsslein, U. Bruhlmann and J.R. Huber, J. Phys. Chem 97 (1993) 989. 161 J.M. Mestdagh, M. Berdah, I. Dimcoli, M. Mons, P. Meynadier, P. d'Olieira, E Piuzzi, J.P. Visticot, C. Jouvet, C. Lardeux-Dedonder, S. Martrenchard-Barra, B. Soep and D. Solgadi J. Chem. Phys. 103 (1995) 1013. [71 M. Nonella, J.R. Huber, A. Untch and R. Schinke, J. Chem. Phys. 91 (1989) 194. 181 A. Untch, R. Schinke, R. Cotting and J.R. Huber, J. Chem. Phys 99(1993) 9553 . 191 O. Benoist d'Azy, E Lahmani, C. Lardeux and D. Solgadi, Chem. Phys. 94 (1984) 247. [ 101 S.A. Reid, J.T. Brandon and H. Reisler, Chem. Phys. Lett. 209 (1993) 22. [ I I I E. Kades, M. R6sslein and J.R. Huber, Chem. Phys. Lett. 209 (1993) 275. [ 121 C.J.S.M. Simpson, P.T. Griffiths and M. Towrie, Chem. Phys. Lett. 234 (1995) 203. [13] C.J.SM. Simpson, P.T. Griffiths, R.L. Lovegrove, P. Matousek and M. Towfie, Chem. Phys. Lett. 246 (1995) 269. [ 14] C.J.S.M. Simpson, P.T. Griffiths, J.M. Curry and M. Towrie, Chem. Phys. Lett. 250 (1996) 342. J 15] W.C. Riley and I.H. McClaren, Rev. Sci. Instrum. 26 (1955) 1150. 1161 C.J.S.M. Simpson, P.T. Gfiffiths, W. Twigger and M. Towrie, in preparation. [ 171 M. Hippler and J. Pfab, Chem. Phys. Lett. 243 (1995) 500.