New vibronic states of NH2 observed in ammonia photolysis

Volume 197, number I,2

CHEMICAL PHYSICS LETTERS

4 September 1992

New vibronic states of NH2 observed in ammonia photolysis J. Schleipen, J.J. ter Meulen Department of Molecular and Laser Physics, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands

L. Nemes Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, I 112 Budapest, Hungary

and M. Vervloet Laboratoire de Photophysique Mokulaire,

VniversitC de Paris-Sud, 91405 Orsay Cedex, France

Received 6 May 1992; in final form 26 June 1992

We report the observation of two high energy vibronic transitions in the NH2 radical, unreported so far. The NH2 radical was produced in a laser photolysis experiment and expanded in a molecular beam machine. Laser excitation and dispersed fluorescence spectra provided term values and rotational constants in good agreement with theoretical predictions.

1. Introduction There has been very extensive research on the electronic transitions of the NH2 radical from the pioneering work of Dressler and Ramsay [ 1 ] extending to recent times [ 2-5 1. Laser excitation and laserinduced fluorescence techniques [3,5-i’] helped simplify the complicated spectrum of the amidogen radical and yielded spectroscopic information for the vibrationally highly excited electronic ground (ii ‘B , ) and excited (A 2A, ) states. Such studies were extended to vibronic transitions excited near 20000 cm-’ ( = 500 nm) involving up to five quanta of vibrational excitation in the v2 bending mode. Electronic absorption spectra were initially photographed between 390 and 830 nm, i.e. up to about 25640 cm-’ [ 11. In addition relative lifetimes and collisional relaxation rates were given for the *A, electronic state [ 6 1. In this Letter, we report observations of high en-

ergy vibronic transitions in the amidogen radical. These results were unexpected by-products of an experiment that was aimed at the production of van der Waals complexes by photolyzing ammonia in Ar buffer gas. In addition to the short-lived triplet lines belonging to the NH radical, there were lines in the spectrum belonging to a long-lived species. This Letter shows that the observation of these latter spectral lines led to the identification of so far unreported parts of the NH2 electronic spectrum, containing v;=lO and v; = 12 vibrational excitations in the A state. The assignment of these spectra could not be based on absorption or emission spectra because no such data were available. Instead the vibronic assignments reported here were based on theoretical calculations by Jungen et al. [ 8,9] and by Duxbury and Dixon [ 111, and by combination differences analysis.

2. Experimental Correspondence to: J. Schleipen, Department of Molecular and Laser Physics, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands.

The NH2 radicals were produced in a molecular jet by photolysis of NHJ. For this purpose a mixture of

0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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1% NH3 in Ar was expanded into vacuum with a backing pressure of 1 atm using a home-built pulsed solenoid valve with a fwhm of 500 us. The vacuum pressure during operation of the valve remained well below 1O-s Torr. Just downstream of the valve opening a channel, 10 mm long and 1 x 1 mm2 in cross section, was mounted through which the gas flows before the expansion takes place. This consisted of two parallel quartz windows separated by stainless steel mounting. An artist’s view of the photolysis apparatus is presented in fig. 1. In order to dissociate the NH3 molecules the output radiation of an ArF excimer laser was slightly focused (f= 50 cm ) through the entrance quartz window. The ArF laser photons have a wavelength of 193 nm which results in a very efficient dissociation of the ammonia molecule. The output power was about 80 mJ in a 15 ns pulse. Inside the photolysis channel the energy density of the 193 nm photons is high enough to dissociate the NH3 and to produce the NH and NH2 radicals. After leaving the capillary the molecules undergo an adiabatic expansion. In this expansion the internal energy of the molecules is converted into translation, which results in a relatively cold NH and NH2 rotational spectrum, simplifying the analysis of the measured spectra. To probe the radicals a second laser was directed perpendicularly onto the molecular beam 50 mm downstream. This second probe laser consisted of a XeCl excimer laser pumped dye

Photolysls

Apparatus

Fig. I. Artist’s impression of the photolysis apparatus. Before the expansion takes place the gas flows through a 10x 1 x 1 mm’ channel, positioned directly behind the nozzle exit. The channel consists of two parallel quartz plates separated by a stainless steel mounting. Through the quartz plates the 193 nm output of an ArF excimer laser is focused, dissociating the NH3 molecules.

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laser, operating on the dye PTP, and was tunable from 332 to 350 nm. The output power was typically 2 mJ/pulse and the bandwidth of the laser pulse was about 0.7 cm-‘. The fluorescence induced by this second probe laser was collected by a mirror and lens assembly and focused on a photomultiplier. In between the LIF collecting optics and the photomultiplier a l/8 m monochromator (Oriel) could be positioned and the LIF light could be studied under dispersion. In this way we were able to obtain both excitation and emission spectra of the radical under study.

3. Experimental results and analysis Laser excitation spectra were recorded between 29200 and 28850 cm-’ with different delay times between photolysis and probe laser. Spectra of shorter delay times show strong triplet features which are easily identifiable as the lines of the O-O and l-l bands of the electronic system A 311+X 3C- of the NH radical. Their precisely known frequencies [ 12 ] were used to calibrate the position of the other lines which do not belong to this system, the resulting precision is believed to be better than 0.4 cm-‘. With longer delay times the NH triplets are no longer dominant in the excitation spectra and the remaining transitions exhibit relative intensities varying with the delay time. Since the lifetime of the NH radical was short compared with the lifetime of the unknown species, we were able to measure both species independently by appropriately adjusting the gate of the boxcar averager. In fig. 2 part of the excitation spectrum is shown. The lines do not form regular branches, some of them are components of narrow doublets which have variable splittings going up to about 6 cm-‘. The irregular spacing between the lines can be relatively large. On the basis of previous studies of ammonia photolysis [ 1,2,13], it could be guessed that the species responsible for this spectrum is NH2, excited by the probe laser to high energy levels which, to our knowledge, have never been reported. For various lines the fluorescence was dispersed under low resolution and the resulting spectra showed the same vibrational progression as in fig. 3. By using previous results [ 3,9], the vibrational analysis

4 September 1992

CHEMICAL PHYSICS LETTERS

Volume 197, number 1,2 LIF SIGNAL

i

..-

LA..” 4

2WOOO

FREQUENCY Icm-‘1

2awo.o

Fig. 2. Excitation spectrum of the NH2 radical in the 28900-29200 cm-’ energy region. Frequency calibration is achieved by identification of NH lines in this spectral region. E.g. the triplet structure at 29200 cm-’ is assigned as the P( 18) transition of the NH radical. Most of the lines could be assigned as rotational transitions in the A *Al (0, 10, 0) K,= I+% *B, (0, 0,O) and A *A, (0, 12, 0) K,= 0-R ‘B, (0,2,0) vibronic bands NH?.

could be carrier out unambiguously,

involving,

(0,

0, O), (0, 1, O), (0, 2, O), (0, 4, 0), (1, 3, O), (0, 6, 0), (1, 4,0), (0, 7,0) and (0, 9,0) of the electronic ground state and (0, 10, 0) (in the bent notation) of the excited state. Moreover, the resolution was high enough to allow the observation of partly resolved rotational structure. When dispersing the fluorescence of the 29048.8 cm-’ excitation, each band is composed of three lines whose frequency combination differences can be related to the combination differences of the rotational energies [ 3 ] between the Ooo,202 and 220 states in each lower vibrational level. According to the C-type selection rule, which gov-

erns the A *A, +-% *B, electronic transition of NH2, the selectively excited upper rotational level must have the rotational assignment 1 ro. The observed intensities in the dispersed fluorescence spectrum agree with calculated transition moments for emission from A ‘A, (0, 10, 0) to the electronic ground state L%2B, [ 9,101. The weakest transitions are predicted to occur for (0, 0, 0), (0, 5, 0) and (0, 8, 0) of the 2 state; the two latter transitions are indeed not observed in the dispersed spectrum of fig. 3. Two other dispersed fluorescence spectra, carried out on the absorption lines at 29069.3 and 29093.6 cm-’ indicate, by doing the same type of analysis, that the ro167

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Volume 197, number 1,2

4 September 1992

-I =LUORESCENCE INTENSITY (0,3,0)

(WO,O) -

(V1”,V2”.V3”)

(1,490) WV’) (0,7,0)

(OAO) I

(O,W)

h

I

I

I

I

I

42ao

340.0

5oao WAVELENGTH 1nm

1

I

580.0

I-

Fig. 3. Dispersed fluorescence spectrum of the I ,o+Ooo (F, tF,)transitioninthe~2A,(0,10,0)K,=l+~2B,(0,0,O)bandofNH,, showing a nice progression in v2. The transitions to the vibrational states (0,0, 0), (0, 50) and (0, 8,O) in the electronic ground state are very weak or even absent in the spectrum because of an almost zero transition moment [ 9, lo]. All bands show rotational structure which can be assigned as a P, Q and R transition from the 1in level in the excited state.

Table 1 Observed line frequencies (in cm-‘) for the NH2 .3,‘Ai (0, 10, 0) K; = I +g *B, (0, 0, 0) transition. The absolute frequencies were calibrated using the accurate line positions of the NH radical [ 121. The experimental accuracy is about 0.4 cm- ’ Frequency (cm-‘)

29093.6 29069.3 29048.4 29047.4 29022.9 29022.3 29015.9 28986.0 28985.1 28960.9 28960.4 28931.7 28930.1 28928.3 28927.3 28899.5 28899.2

Obs. - talc. (cm-‘)

0.1 -0.4 0.5 0.0 0.4 0.3 0.0 0.1 -0.2 -0.6 -0.3 0.5 -0.1 0.1 0.1 0.2 0.5

Transition J&-J&:

spin assignment

3i2-2n2 2,,-lo,

F,tF,+F,tF, F,tF, +F2tF2

1to-000 110-000

F,tF, Fz+Fz

l,,-lo1 l,,-lo1 212~.&2

FieFi Fz+Fz F,tF,+F2+FZ

110-202

F,+F,

1m-&2

F,+F,

212-220

F,+F,

2,2-220

Fz+-Fz

110-220

F,tF,

l io-220 111-221

Fz+Fz F,+-F,

111-221

F2+F2

212-322 2,2-322

F,+Fi F,+F,

tational assignments of the upper levels are 2,, and 3,2 respectively. Other transitions with K:, = 1 were identified on the basis of combination differences between the well-known energies of the rotational levels of z *B, (0, 0, 0) [ 21. However, the precision 168

on the line frequencies is not high enough to decide on the spin assignment. It is frequently observed for NH2, that the strongest line of a doublet is the F1 t F, component, while the weaker one is the F2+F2 transition. Following this observation, the spin assignment was arbitrarily given according to this criterion. The assignments resulting from this first part of analysis are reported in table 1. A second set of lines in the higher frequency range do not belong to the Kg = 1 subband. They have medium intensity and their intensities do not follow the same variation as the Kg = 1 lines when the delay time varies. Their intensity variation indicates that they are emitted from a vibronic level of longer lifetime. Only one fluorescence from this set of lines was dispersed when the excitation frequency was 29 139.4 cm-‘. The resulting spectrum shows several bands which have no resolved rotational structure; moreover they exhibit roughly constant width. This indicates that only one K; sublevel is involved in each branch. Consequently, the upper level must have Kg = 0 in order to follow the selection rule AK, = + 1 for a C-type electronic transition. The vibrational analysis could be carried out by comparing the frequency differences between the bands and the results of ref. [ 3 1. The observed bands involve (0,2,0), (0, 3,0), (0,4,0), (0, 5,0), (0,6, O), (134, O), (0, 7, 0), ( 1, $0) and (0, 8,0) of the ground state. From the calculations of Jungen et al. [9] we concluded

CHEMICAL PHYSICS LETTERS

Volume 197, number 1,2

that the upper state is (0, 12, 0). The lowest levels (0, 0, 0) and (0, 1, 0) are not observed in the dispersed fluorescence spectrum. This is in agreement with calculations of the transition moments for emission from A ‘A, (0, 12, 0 ) [ 10 ] which predict very weak intensities for the (0, 12, 0) --t (0, 0, 0) and (0, 1,O) bands. It was concluded that the lower vibronic state of the 29139.4 cm-’ excitation is (0, 2, 0) K!! = 1. With this information, by using combination differences obtained from ref. [ 41, the strongest remaining lines were assigned. The result of this rotational analysis is reported in table 2. The term values of the upper rovibronic levels were calculated by adding the line frequencies to the rovibronic energies of the lower levels [ 2,4]. Next they were used in a least-squares determination of the parameters which are defined in the modified version of the Hill and van Vleck equation given in eq. ( 1) of ref. [ 8 1. The results of this determination are given in table 3. The vibronic energies T,,are very close Table 2 Observed line frequencies (in cm-’ ) for the NH2 A *A, (0, 12, 0) K:, =O+i( *B1 (0, 2, 0) transition. The absolute frequencies were calibrated using the accurate line positions of the NH radical [ 121. The experimental accuracy is about 0.4 cm-’ Frequency (cm-‘)

29139.4 29139.0 29086.4 29173.7 29142.2 29055.6 29189.6 29150.8 29027.1

Obs. - talc. (cm-‘)

0.6 1.2 0.7 -1.0 -0.9 -1.1 0.4 0.3 0.4

Transition J&;-J;;:,::

spin assignment

101-l II lor-111 101-211 202-f 10 202-212

F,+F, Fz+Fz F,tF,+FZtF2 F,+F,+F2tF2 F,+F,+F2+F2 F,+F,+F2tF2 F,+F,+F2tF2 F,+F,+F2tF2 F,+F,+F,+F*

202-b

303-211 303-313 303-413

Table 3 Molecular constant (in cm- ’ ) for the vibronic states IT(0, 10,O) and X(0, 12, 0) of NH2 in the A’A, electronic state. The constants used are defined in ref. [ 81

T Y,k B Y.*

A”v.k 4”.*

K,= 1 (0, 10,O)

K,=O

29035.2(2) 9.71(5) 0.65(40) 2.14(5)

32115.9(5) 10.35(6)

(0, 12,O)

4 September 1992

to the theoretical predictions of Jungen et al. [9], where they are calculated at 32124.82 and 29043.26 cm-’ for (0, 12,0) Kg=0 and (0, 10,O) K6=1, respectively, in A 2A,. No calculations were available for the rotational constants involved in this study. Jungen et al. [ 81 calculated the rotational constants & 1_(B+ C), in the A 2A, K= 0 and K= 1 states up to q = 9 (bent notation). By extrapolating these results to v2= 10 and v2= 12 our experimental B value for the K,= 1 (0, 10, 0) state is in good agreement with theory. However, for the K,= 0 (0,12, 0) state the discrepancy yields about 5% of the experimental value. The reason for this discrepancy might be the lack of experimental data (only three rotational levels in this electronically excited state are observed) in combination with a possible perturbation of the J&,: = 3,,3 level, causing an overestimate of the experimental B value. There are still several lines in the spectrum shown in fig. 2 which are not identified yet. Especially at z 29 120 cm-’ a bunch of lines show up which, to our opinion, belong to the RR branch of a A vibronic band. However, identification of these lines requires a thorough reinvestigation of the excitation spectrum, including a dispersion of the observed fluorescence.

4. Summary In this Letter, we report the observation of two new vibronic bands of the NH2 radical produced in a photolysis experiment. Excitation and dispersion spectra yielded the NH2 molecular constants for the A2A, (0, 10, 0) K,=l and A2A, (0, 12, 0) K,=O vibronic states. The term values as well as the transition moment matrix elements for the vibronic transitions under study are in perfect agreement with the theoretical values of Jungen et al. [ 9,101, indicating the accuracy of their calculations. It is interesting to notice that while our experiments and similar ones by Japanese authors [ 71 led to rotationally very cold NH2, analogous experiments by Dixon et al. [ 5 ] resulted in exclusively high Kg excitations. The latter is furthermore confirmed by H-atom TOF studies [ 141 on the NH3 photofragmentation process which show that preferen169

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tially high K, levels are excited (up to K, = 20) in the NH2 product. In all three studies free jets of NH3 photolysis products were used. The main difference between our experimental method and the one of Dixon et al. [ 51 is that we expanded a 1% NH3 in Ar mixture with a backing pressure of 700-800 Torr, while Dixon et al. used a pure NH3 sample with a backing pressure of 150-250 Torr. Furthermore, in the Dixon experiment the probe and photolysis laser were counterpropagating and hence the NH2 radical was probed immediately after dissociation of the NH3. In our case the NH2 is first produced inside the photolysis channel and after that it expands into vacuum seeded in an Ar environment giving rise to the efficient rotational cooling observed in our spectra. In view of the serendipity of the presently reported observations it appears that repeated laser excitation and LIF spectroscopic measurements in and above the 340 nm energy range might reveal interesting features. Not only may the NH3 photofragmentation mechanism be better understood, it will also provide us with new information on the high energy states of the very important amidogen radical.

Acknowledgement The authors are greatly indebted to Dr. Ch. Jungen for his theoretical help, including personal communications, in the analysis of the spectra. LN would like to acknowledge the financial support of the

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LETTERS

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1992

Netherlands Foundation for Scientific Research (NWO) and the Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, for the leave of absence.

References

[ 1 ] K. Dressier and D.A. Ramsay, Phil. Trans. Roy. Sot. A 25 1 (1959) 553. [ 21 S.C. Ross, F.W. Birss, M. Vervloet and D.A. Ramsay, J. Mol. Spectry. 129 (1988) 436. [3] M. Vervloet, Mol. Phys. 63 (1988) 433. [ 41 A.R.W. McKellar, M. Vervloet, J.B. Burkholder and C.J. Howard, J. Mol. Spectry. 142 ( 1990) 3 19. [ 51R.N. Dixon, S.J. Irving, J.R. Nightingale and M. Vervloet, J. Chem. Sot. Faraday Trans. 87 (1991) 2121. [ 61 J.B. Halpern, G. Hancock, M. Lenzi and K.H. Welge, J. Chem. Phys. 63 (1975) 4808. [ 71 S. Mayama, S. Hiraoka and K. Obi, J. Chem. Phys. 80 (1984) 7. [8] Ch. Jungen, K.-E. Hallin and A.J. Merer, Mol. Phys. 40 ( 1980) 65. [9] Ch. Jungen, K.-E. Hallin and A.J. Merer, Mol. Phys. 40 (1980) 25. [lo] Ch. Jungen, private communication. [ 111 G. Duxbury and R.N. Dixon, Mol. Phys. 43 (1981) 255. [ 121 C.R. Brazier, R.S. Ram and P.F. Bernath, J. Mol. Spectry. 120 (1986) 381. [ 13 ] J.W.C. Johns, D.A. Ramsay and S.C. Ross, Can. J. Phys. 54 (1976) 1804. [ 141 J. Biesner, L. Schnieder, J. Schmeer, G. Ahlers, X. Xie, K.H. Welge, M.N.R. Ashforld and R.N. Dixon, J. Chem. Phys. 88 (1988) 3607.