- Email: [email protected]
Dispersed emission spectrum of NH2 (Ã 2A1) in the ultraviolet laser photolysis of HN3 and the mechanism of formation
Volume 204, number 1,2
CHEMICAL PHYSICS LETTERS
12 March 1993
Dispersed emission spectrum of NH2 (A ‘Al ) in the ultraviolet laser photolysis of HN3 and the mechanism of formation Katsuyoshi Yamasaki, Akihiro Watanabe, Ikuo Tokue and Yoshio Ito Department of Chemistry, Niigata University, bnocho, Ikarashi, Niigata 950-21, Japan
Received 24 November 1992; in final form 9 December 1992
The chemiluminiscent reaction following the ultraviolet (266 nm) photolysis of HN, has been studied. A dispersed emission spectrum was recorded, which is, to the best of our knowledge, the fast direct identification of the origin of the chemiluminescence. The emissive species was assigned to vibronically excited NH2. The observed vibrational distribution in the NHz(A’A,) was not consistent with the reaction mechanism previously suggested (NH (a ‘A,IJ~0) + HN,). A modified mechanism including vibrationally excited NH( a ‘A) has been proposed on the basis of energetics and kinetics in terms of the emission.
1. Introduction In their pioneering work, McDonald and co-workers [l-3] observed visible emission in the 266 nm photolysis of HNJ using optical filters. They suggested the electronic transition NH2(A *A1+% *B, ) for the emission only by comparison with its absorption wavelength. They also proposed a reaction, NH(a’A, v=O)+HN3+NH2(.%*A,)+Nj, on the basis of the heat of reaction. Piper et al. [ 41 observed time profiles not onIy of the visible emission but also of laser-induced fluorescence (LIF) from NH (c ‘l7 + a ‘A, v= 0). They reported both profiles were consistent with the mechanism proposed by McDonald and co-workers, although their rate constant for the reaction was about two times larger than that measured by McDonald and co-workers. Thereafter the mechanism has been accepted with little doubt; no study has obtained conclusive evidence for the origin of the emission by means of direct spectroscopic techniques. The overall rate constant for the reaction NH(a ‘A) +HN3 has also been measured [ 5-91; however, there have been few studies on the reaction products, particularly on the electronically excited states. In the present study, a dispersed emission spectrum was observed; the emissive species was 106
identified as vibronically excited NH*. The internal state distribution was not necessarily consistent with the reaction mechanism accepted so far. We have proposed a modified mechanism including vibrationally excited NH (a ‘A) from spectroscopic and kinetic information on the emission.
2. Experimental Fig. 1 shows a schematic diagram of the apparatus. The sample gas, which was slowly flowed in a fluorescence cell, was irradiated with a 266 nm beam (0.2-5 mJ pulse-‘, 4-10 Hz, 6 mm in diameter) from a Nd3+ :YAG laser (Continuum YG660-20). The pressure of the gas was measured with a capacitance manometer (Baratron 122AA). In the experiment to record dispersed spectra, emission from the observation region was collected through a long-pass filter (Toshiba Y-43) and was focused on the entrance slit of a monochromator (Nikon P-250). The signal from a photomultiplier (Hamamatsu R928 ) mounted on the exit slit was fed into a gated integrator (Stanford SR250). In order to corrkct for the shot-to-shot fluctuations, undispersed emission was monitored with another photomultiplier (Hamamatsu lF28) through a filter (Toshiba R-54) at the
0009-2614~93/$06.00 Q 1993 Elsevjer Science Publishers B.V. All rights reserved.
Volume 204, number 1,2
CHEMICAL PHYSICS LETTERS
12 March 1993
Monochromator
Fig. 1. A schematic diagram of the apparatus. The laser beam was reflected with a dichroic mirror. PMT: photomultiplier; I/O: computer interface.
opposite side of the monochromator. The signal from the lP28 was averaged with a home-made gated integrator. Both signals were sent to a computer and stored on a disk. In the experiments to observe temporal profiles of the dispersed emission, the signal from the R928 was digitized with a home-madetransient memorywhose time resolution was 40 ns. Signals from 15000-30000 laser pulses were averaged to obtain a good S/N ratio. The time-dependent profiles were then analyzed with a double exponential non-linear least-squares tit. Hydrogen azide, HN3, was synthesized in vacua by the reaction between sodium azide (NaN3) and excess stearic acid (C,,H&OOH) [ lo]. Only a small amount (less than 5 mmol) of HN3 was prepared in each synthesis to prevent unexpected explosion.
3. Results and discussion 3.1. Spectroscopic data Fig. 2 shows a dispersed emission spectrum which is the first such observation following the UV photolysis and subsequent reactions of HNJ. A signifi-
Nti#‘A,.
E
V; = 16 15 16 13 12 11
1
0h$0-~'8,~000) 10
9
I
1
I
400
500
6ocl
6
7
6
I
700
5
4
I
800
Wavelength/nm
Fig. 2. Dispersed emission spectrum of the 266 nm photolysis of HN,. Pressure of HN, was 150mTorr with no buffer gas; spectral resolution was 3 nm (fwhm). The assignments are appropriate to the linear configuration for the L’A, state [ 111. The onset at about 420 nm is due to the filter used.
cant progression appears along with a broad background. Most of the peaks were assigned to the NH,(A2A,(0, u;, 0)-82B,(0,0,0)). transition The broad background results from congestion of many weak vibronic sub-bands [ 1l- 131. The spectrum shows the production of bending vibrational levels up to at least u; = 13. McDonald et al. [ l-31 proposed that the
107
Volume 204, number 1,2
CHEMICAL
NH,(A 2A,)
was produced in the reaction NH(a’A,v=O)+HN,-+NH2(A2A,)+Nj. Assuming that all the exothermicity of the reaction is deposited into NH2, we can estimate the maximum vibrational quantum number from the heats of formation of all species related to the reaction, The AHgs, for HN,(z’A’), NH(%‘C-), and NH2 (% ‘B, ) have been determined precisely: 300.5?0.3 kJ mol-’ [ 141; 3572 1 kJ mol-’ [ 151; 192+ 1 kJ mol-’ [ 151, respectively. O-O transition energies are known for NH (a ‘A-2 3Z-) and NH2(A2A’-%2B’): 12589 cm-’ [ 161; 10249 cm-’ [ 171. For NS, unfortunately, an accurate heat of formation is not available. Although M&(N3) = 417?21kJmol-’ [18] hasbeenadoptedforalong time, recent reliable reports using various techniques recommended higher values: 469 + 2 1 kJ mol-’ from ion cyclotron resonance of NT [ 191; ~475.3 kJ mol-’ in the study of the reaction Ft HN3-, HF(&4)+N, [20]; 457.1 kJ mol-’ from quantum-mechanical calculation [ 2 11. These values give the maximum NH1( A 2A,) vibrational number v;,,=O-5, > 2, and 4, respectively. The high vibrational levels observed in the present experiment have more energy than the highest level estimated from the mechanism accepted so far. There is also another mechanism for the production ofthe NH,(A 2Al-g ‘B,) emission in the presence of HN3. Kajimoto et al. [22] observed emission from NH2 produced in the reaction H+ HN, +NH, (A 2Al, vi < 15 ) -t NZ. Thus, the hydrogen atom is also a candidate for a reactant. A reaction mechanism consistent with observed emission will be discussed in section 3.3. 3.2. Kinetic data The emission intensity varied linearly with the laser fluence, irrespective of the observation wavelengths. The photon must be associated with the dissociation of HN3. NH(a ‘A) was believed to be a dominant product in the photolysis of HN3 over the wavelengths 240-3 10 nm until recently [ 1,2,23,24]. Gericke et al. [25], however, have measured the quantum yield for another possible path, to form H +N3, using vacuum UV LIF: 0.24kO.05 at 248 nm and 0.15? 0.02 at 193 nm. Since the upper potential of HN3 in absorption at 248 and 266 nm is the same
108
12 March 1993
PHYSICS LETTERS
electronic state (A’A”) [IO], the H+N3 channel might be open in the photolysis at both wavelengths with a similar quantum yield. As stated in section 3.1, hydrogen atom also reacts with HN3 and proNH2(A2A,). excited vibrationally duces NH2 (A ‘A’) decays through quenching by HN3 and radiation, irrespective of how the NH2 (A ‘A’) is produced. A reaction scheme thus can be written as follows: HN,+hv(266nm)+NH(a’A)+N2,
(1)
HN+h~(266 nm)+H+N,,
(2)
XtHN3+NH2(A2A’)tY,
(3a)
X t HNS-products ,
(3b)
NH,(A ‘A’) +HN,+products ,
(4)
NH,(A2A’)+NH2(z2Bl)
(5)
thv’,
where X denotes NH (a ‘A) or H; Y denotes N3 or N2, respectively. As seen in the scheme, NH2 (A 2Al) is an intermediate product in the consecutive intensity the emission thus, reactions; ( a [ NH2 (A *A,) ] ) shows a rise and fall. It should be noted that a faster process always governs the rise and a slower one relates to the fall. In the present experiment, the rise must be related to the quenching of NH2(A ‘Al ) because of its extraordinarily large rate constant [26,27]; the fall corresponds to the overall decay of the reactant X which produces NH,(A ‘A’) in the reaction with HN3. The concentration of X was estimated to be sufficiently lower than that of HNS; the reactions (3) and (4)) therefore, proceed under pseudo-first-order conditions. Fig. 3, the first-order decay rate versus concentrations of HNs, gives the overall rate constant k3( = kja t k3b). Profiles at different wavelengths (520 and 600 nm) were observed, where the emissions were due mainly to the transitions from NH,(x ‘A’) vi = 12 and 9. Rate constants were obtained to be k,(520 nm)=(l.lfO.l)xlO-‘O and k,(600 nm)=(0.92~0.1)~10-I0 cm3 molecule-’ s-‘. The errors denote 2a in the least-squares analysis. The rate constants measured at the two NH2 (A ‘A, -3 ‘B’) emission wavelengths are close in value.
Volume204,number I ,2
CHEMICAL. PHYSICS LETTERS
Fig. 3. Dependence of the first-order decay rate (,&)
12 March 1993
of the emission on the concentration of HNI, observed at 520 nm (a) and 600
nm (b). The lines are drawn by the weighted least-%&es fit.
3.3. Reaction mechanism Le Bras and Combourieu [ 28 ] determined a rate constant for the reaction H+HN,+products using a discharge-flow method coupled with mass spectrometry. An Arrhenius expression for the rate constant was given: 2.54x lo-l1 exp[ (-4600/cal mol-‘)/RT] cm3 molecule-’ s-‘. The rate constant k3 obtained in section 3.2 is much larger than that for the reaction H+HN, at any temperature. The large difference shows that the NH,(A2A,) observed in the present work cannot be produced by the H+HN, reaction. For another candidate, X =NH( a ‘A), on the other hand, several groups have measured the overall rate constant from direct observation of NH( a ‘A) decay [ 5-91. The values are within the range of (l.O1.4) x 10-i’ cm3 molecule-’ s-r, which is almost the same magnitude as k, in the present study. The agreement strongly supports a mechanism that the NH2 (A *Al) observed at all wavelengths is produced in the reaction between NH (a ‘A) and HN,. The remaining question is the production of highly vibrationally excited NH2( A *A1) which cannot be ascribed to the reaction NH(a ‘A, v=O) +HNJ --) NH2 (A *A, ) + N3. The formation of the high vibrational levels of NH2 (A 2A,) might result from a reaction of NH (a ‘A) which is excited vibrationally. Nelson and McDonald [29] have drawn a conclu-
sion on energy partitioning in the 266 nm dissociation of HN3. According to their summary, nascent NH (a ‘A) is rotationally cool but hot in translation and vibration. In our preliminary experiments, the profile of the dispersed spectrum did not change even in the presence of an excess amount of He, which suggests that translational excitation is not necessarily a cause of the production of NH,(A *A,) in the high vibrational levels. Nelson and McDonald [29] reported a nascent NH (a ‘A, u) vibrational distribution: 1.O/ ( 1.1 i: 0.3)/(0.8f0.3)/(0.9+0.5) for v=O/l/2/3. Adams and Pasternack [ 301 also observed vibrational excitation of NH( a ‘A, Y=O-2) in the same (HNJ 266 nm) photolysis. Moreover, Hack and Mill [ 91, using the LIF technique, have recently shown that the overall rate constants for the reaction NH( a ‘A, v= O-2) + HN3 have little dependence on the vibrational levels of NH( a ‘A). In the present study, the rate constant k3 measured at different NH2 wavelengths are close to each other, which is consistent with the results obtained by the LIF technique. All findings in the present experiment give rise to the conclusions that the visible emission originates from vibronically excited NH*, that all vibrational levels of NH2 (A *A,) are produced in the reaction of NH( a ‘A) with HNs, and that highly vibrationally excited NH2 (A *A1) is produced from vibrationally excited NH(a ‘A, u). 109
Volume 204, number 1,2
CHEMJCAL PHYSICS LETTERS
Acknowledgement
The authors are indebted to Okitsugu Kajimoto (Kyoto University), Tomoko Kimura and Aki Tanaka (Niigata University) for their valuable help to perform the experiments.
References [ 1 ] J.R. McDonald, R.G. Miller and A.P. Baronavski, Chem. Phys. Letters 51 (1977) 57. [2] A.P. Baronavski, R.G. Miller and J.R. McDonald, Chem. Phys. 30 (1978) 119. [3] J.R. McDonald, R.G. Miller and A.P. Baronavski, Chem. Phys. 30 (1978) 133. [4] LG. Piper, R.H. Krech and R.L. Taylor, J. Chem. Phys. 73 (1980) 791. [5] F. Rohrer and F. Stuhl, Chem. Phys. Letters 111(1984) 234. [6] F. Freitag, F. Rohrer and F. Stuhl, J. Phys. Chem. 93 (1989) 3170. [7] J.W. Cox, H.H. Nelson and J.R. McDonald, Chem. Phys. 96 (1985) 175. [ 81 W. Hack and A. Wilms, Z. Physik. Chem. 161 ( 1989) 107. [9] W. Hack and Th. Mill, J. Phys. Chem. 95 (1991) 4712. [lo] J.R. McDonald,J.W. Rabalaisand S.P. McGlynn, J. Chem. Phys. 52 (1970) 1332. [ I1 ] K. Dressler and D.A. Ramsay, Phil. Trans. Roy. Sot. (London)A251 (1959) 553. I 121S.C. Ross and F.W. Birss, M. Vervloet and D.A. Ramsay, J. Mol. Spectry. 129 (1988) 436.
110
I2 March 1993
[ 131F.W. Birss, M.-F. Merienne-Lafore, D.A. Ramsay and M. Vervloet, J. Mol. Spectry. 85 ( 1981) 493. [ 141D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Chumey and R.L. Nuttall, J. Phys. Chem. Ref. Data 11 Suppl. 2 (1982). [ 151 W.R. Anderson, J. Phys. Chem. 93 (1989) 530. [ 161K.P. Huber and G. Herzberg, Molecular spectra and molecular structure, Vol: 4. Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1979). [ 171 G. Herzberg, Molecular spectra and molecular structure, Vol. 3. Electronic spectra and electronic structure of polyatomic molecules (Van Nostrand Reinhold, New York, 1966). [ 181T.C. Clark and M.A.A. Clyne, Trans. Faraday Sot. 66 (1970) 877. [ 191 M.J. Pellerite, R.L. Jackson and J.I. Brauman, J. Phys. Chem. 85 (1981) 1624. [201 J. Habdas, S. Wategaonkar and D.W. Setser, J. Phys. Chem. 91 (1987) 451. [21] J.M.L.MartinandJ.P. Fraqois, J.Chem.Phys.93 (1990) 4485. [ 2210. Kajimoto, T. Kawajiri and T. Fueno, Chem. Phys. Letters 76 (1980) 315. 1231B.M. Dekoven and A.P. Baronavski, Chem. Phys. Letters 86 (1982) 392. [24] F. Rohrer and F. Stuhl, J. Chem. Phys. 88 (1988) 4788. [ 251 K.-H. Gericke, M. Lock and F.J. Comes, Chem. Phys. Letters 186 (1991) 427. [ 261 J.B. Halpem, G. Hancock, M. Lenzi and K.H. Welge, J. Chem. Phys. 63 (1975) 4808. [27] LJ. Wysong, J.B. Jeffries and D.R. Crosley, J. Chem. Phys. 93 (1990) 237. [28] G. Le Bras and J. Combourieu, Intern. J. Chem. Kinetics 559 (1973) 559. [29] H.H. Nelson and J.R. McDonald, J. Chem. Phys. 93 (1990) 8717. [ 301 J.S. Adams and L. Pastemack, J. Phys. Chem. 9 5 (1991) 2975.
CHEMICAL PHYSICS LETTERS
12 March 1993
Dispersed emission spectrum of NH2 (A ‘Al ) in the ultraviolet laser photolysis of HN3 and the mechanism of formation Katsuyoshi Yamasaki, Akihiro Watanabe, Ikuo Tokue and Yoshio Ito Department of Chemistry, Niigata University, bnocho, Ikarashi, Niigata 950-21, Japan
Received 24 November 1992; in final form 9 December 1992
The chemiluminiscent reaction following the ultraviolet (266 nm) photolysis of HN, has been studied. A dispersed emission spectrum was recorded, which is, to the best of our knowledge, the fast direct identification of the origin of the chemiluminescence. The emissive species was assigned to vibronically excited NH2. The observed vibrational distribution in the NHz(A’A,) was not consistent with the reaction mechanism previously suggested (NH (a ‘A,IJ~0) + HN,). A modified mechanism including vibrationally excited NH( a ‘A) has been proposed on the basis of energetics and kinetics in terms of the emission.
1. Introduction In their pioneering work, McDonald and co-workers [l-3] observed visible emission in the 266 nm photolysis of HNJ using optical filters. They suggested the electronic transition NH2(A *A1+% *B, ) for the emission only by comparison with its absorption wavelength. They also proposed a reaction, NH(a’A, v=O)+HN3+NH2(.%*A,)+Nj, on the basis of the heat of reaction. Piper et al. [ 41 observed time profiles not onIy of the visible emission but also of laser-induced fluorescence (LIF) from NH (c ‘l7 + a ‘A, v= 0). They reported both profiles were consistent with the mechanism proposed by McDonald and co-workers, although their rate constant for the reaction was about two times larger than that measured by McDonald and co-workers. Thereafter the mechanism has been accepted with little doubt; no study has obtained conclusive evidence for the origin of the emission by means of direct spectroscopic techniques. The overall rate constant for the reaction NH(a ‘A) +HN3 has also been measured [ 5-91; however, there have been few studies on the reaction products, particularly on the electronically excited states. In the present study, a dispersed emission spectrum was observed; the emissive species was 106
identified as vibronically excited NH*. The internal state distribution was not necessarily consistent with the reaction mechanism accepted so far. We have proposed a modified mechanism including vibrationally excited NH (a ‘A) from spectroscopic and kinetic information on the emission.
2. Experimental Fig. 1 shows a schematic diagram of the apparatus. The sample gas, which was slowly flowed in a fluorescence cell, was irradiated with a 266 nm beam (0.2-5 mJ pulse-‘, 4-10 Hz, 6 mm in diameter) from a Nd3+ :YAG laser (Continuum YG660-20). The pressure of the gas was measured with a capacitance manometer (Baratron 122AA). In the experiment to record dispersed spectra, emission from the observation region was collected through a long-pass filter (Toshiba Y-43) and was focused on the entrance slit of a monochromator (Nikon P-250). The signal from a photomultiplier (Hamamatsu R928 ) mounted on the exit slit was fed into a gated integrator (Stanford SR250). In order to corrkct for the shot-to-shot fluctuations, undispersed emission was monitored with another photomultiplier (Hamamatsu lF28) through a filter (Toshiba R-54) at the
0009-2614~93/$06.00 Q 1993 Elsevjer Science Publishers B.V. All rights reserved.
Volume 204, number 1,2
CHEMICAL PHYSICS LETTERS
12 March 1993
Monochromator
Fig. 1. A schematic diagram of the apparatus. The laser beam was reflected with a dichroic mirror. PMT: photomultiplier; I/O: computer interface.
opposite side of the monochromator. The signal from the lP28 was averaged with a home-made gated integrator. Both signals were sent to a computer and stored on a disk. In the experiments to observe temporal profiles of the dispersed emission, the signal from the R928 was digitized with a home-madetransient memorywhose time resolution was 40 ns. Signals from 15000-30000 laser pulses were averaged to obtain a good S/N ratio. The time-dependent profiles were then analyzed with a double exponential non-linear least-squares tit. Hydrogen azide, HN3, was synthesized in vacua by the reaction between sodium azide (NaN3) and excess stearic acid (C,,H&OOH) [ lo]. Only a small amount (less than 5 mmol) of HN3 was prepared in each synthesis to prevent unexpected explosion.
3. Results and discussion 3.1. Spectroscopic data Fig. 2 shows a dispersed emission spectrum which is the first such observation following the UV photolysis and subsequent reactions of HNJ. A signifi-
Nti#‘A,.
E
V; = 16 15 16 13 12 11
1
0h$0-~'8,~000) 10
9
I
1
I
400
500
6ocl
6
7
6
I
700
5
4
I
800
Wavelength/nm
Fig. 2. Dispersed emission spectrum of the 266 nm photolysis of HN,. Pressure of HN, was 150mTorr with no buffer gas; spectral resolution was 3 nm (fwhm). The assignments are appropriate to the linear configuration for the L’A, state [ 111. The onset at about 420 nm is due to the filter used.
cant progression appears along with a broad background. Most of the peaks were assigned to the NH,(A2A,(0, u;, 0)-82B,(0,0,0)). transition The broad background results from congestion of many weak vibronic sub-bands [ 1l- 131. The spectrum shows the production of bending vibrational levels up to at least u; = 13. McDonald et al. [ l-31 proposed that the
107
Volume 204, number 1,2
CHEMICAL
NH,(A 2A,)
was produced in the reaction NH(a’A,v=O)+HN,-+NH2(A2A,)+Nj. Assuming that all the exothermicity of the reaction is deposited into NH2, we can estimate the maximum vibrational quantum number from the heats of formation of all species related to the reaction, The AHgs, for HN,(z’A’), NH(%‘C-), and NH2 (% ‘B, ) have been determined precisely: 300.5?0.3 kJ mol-’ [ 141; 3572 1 kJ mol-’ [ 151; 192+ 1 kJ mol-’ [ 151, respectively. O-O transition energies are known for NH (a ‘A-2 3Z-) and NH2(A2A’-%2B’): 12589 cm-’ [ 161; 10249 cm-’ [ 171. For NS, unfortunately, an accurate heat of formation is not available. Although M&(N3) = 417?21kJmol-’ [18] hasbeenadoptedforalong time, recent reliable reports using various techniques recommended higher values: 469 + 2 1 kJ mol-’ from ion cyclotron resonance of NT [ 191; ~475.3 kJ mol-’ in the study of the reaction Ft HN3-, HF(&4)+N, [20]; 457.1 kJ mol-’ from quantum-mechanical calculation [ 2 11. These values give the maximum NH1( A 2A,) vibrational number v;,,=O-5, > 2, and 4, respectively. The high vibrational levels observed in the present experiment have more energy than the highest level estimated from the mechanism accepted so far. There is also another mechanism for the production ofthe NH,(A 2Al-g ‘B,) emission in the presence of HN3. Kajimoto et al. [22] observed emission from NH2 produced in the reaction H+ HN, +NH, (A 2Al, vi < 15 ) -t NZ. Thus, the hydrogen atom is also a candidate for a reactant. A reaction mechanism consistent with observed emission will be discussed in section 3.3. 3.2. Kinetic data The emission intensity varied linearly with the laser fluence, irrespective of the observation wavelengths. The photon must be associated with the dissociation of HN3. NH(a ‘A) was believed to be a dominant product in the photolysis of HN3 over the wavelengths 240-3 10 nm until recently [ 1,2,23,24]. Gericke et al. [25], however, have measured the quantum yield for another possible path, to form H +N3, using vacuum UV LIF: 0.24kO.05 at 248 nm and 0.15? 0.02 at 193 nm. Since the upper potential of HN3 in absorption at 248 and 266 nm is the same
108
12 March 1993
PHYSICS LETTERS
electronic state (A’A”) [IO], the H+N3 channel might be open in the photolysis at both wavelengths with a similar quantum yield. As stated in section 3.1, hydrogen atom also reacts with HN3 and proNH2(A2A,). excited vibrationally duces NH2 (A ‘A’) decays through quenching by HN3 and radiation, irrespective of how the NH2 (A ‘A’) is produced. A reaction scheme thus can be written as follows: HN,+hv(266nm)+NH(a’A)+N2,
(1)
HN+h~(266 nm)+H+N,,
(2)
XtHN3+NH2(A2A’)tY,
(3a)
X t HNS-products ,
(3b)
NH,(A ‘A’) +HN,+products ,
(4)
NH,(A2A’)+NH2(z2Bl)
(5)
thv’,
where X denotes NH (a ‘A) or H; Y denotes N3 or N2, respectively. As seen in the scheme, NH2 (A 2Al) is an intermediate product in the consecutive intensity the emission thus, reactions; ( a [ NH2 (A *A,) ] ) shows a rise and fall. It should be noted that a faster process always governs the rise and a slower one relates to the fall. In the present experiment, the rise must be related to the quenching of NH2(A ‘Al ) because of its extraordinarily large rate constant [26,27]; the fall corresponds to the overall decay of the reactant X which produces NH,(A ‘A’) in the reaction with HN3. The concentration of X was estimated to be sufficiently lower than that of HNS; the reactions (3) and (4)) therefore, proceed under pseudo-first-order conditions. Fig. 3, the first-order decay rate versus concentrations of HNs, gives the overall rate constant k3( = kja t k3b). Profiles at different wavelengths (520 and 600 nm) were observed, where the emissions were due mainly to the transitions from NH,(x ‘A’) vi = 12 and 9. Rate constants were obtained to be k,(520 nm)=(l.lfO.l)xlO-‘O and k,(600 nm)=(0.92~0.1)~10-I0 cm3 molecule-’ s-‘. The errors denote 2a in the least-squares analysis. The rate constants measured at the two NH2 (A ‘A, -3 ‘B’) emission wavelengths are close in value.
Volume204,number I ,2
CHEMICAL. PHYSICS LETTERS
Fig. 3. Dependence of the first-order decay rate (,&)
12 March 1993
of the emission on the concentration of HNI, observed at 520 nm (a) and 600
nm (b). The lines are drawn by the weighted least-%&es fit.
3.3. Reaction mechanism Le Bras and Combourieu [ 28 ] determined a rate constant for the reaction H+HN,+products using a discharge-flow method coupled with mass spectrometry. An Arrhenius expression for the rate constant was given: 2.54x lo-l1 exp[ (-4600/cal mol-‘)/RT] cm3 molecule-’ s-‘. The rate constant k3 obtained in section 3.2 is much larger than that for the reaction H+HN, at any temperature. The large difference shows that the NH,(A2A,) observed in the present work cannot be produced by the H+HN, reaction. For another candidate, X =NH( a ‘A), on the other hand, several groups have measured the overall rate constant from direct observation of NH( a ‘A) decay [ 5-91. The values are within the range of (l.O1.4) x 10-i’ cm3 molecule-’ s-r, which is almost the same magnitude as k, in the present study. The agreement strongly supports a mechanism that the NH2 (A *Al) observed at all wavelengths is produced in the reaction between NH (a ‘A) and HN,. The remaining question is the production of highly vibrationally excited NH2( A *A1) which cannot be ascribed to the reaction NH(a ‘A, v=O) +HNJ --) NH2 (A *A, ) + N3. The formation of the high vibrational levels of NH2 (A 2A,) might result from a reaction of NH (a ‘A) which is excited vibrationally. Nelson and McDonald [29] have drawn a conclu-
sion on energy partitioning in the 266 nm dissociation of HN3. According to their summary, nascent NH (a ‘A) is rotationally cool but hot in translation and vibration. In our preliminary experiments, the profile of the dispersed spectrum did not change even in the presence of an excess amount of He, which suggests that translational excitation is not necessarily a cause of the production of NH,(A *A,) in the high vibrational levels. Nelson and McDonald [29] reported a nascent NH (a ‘A, u) vibrational distribution: 1.O/ ( 1.1 i: 0.3)/(0.8f0.3)/(0.9+0.5) for v=O/l/2/3. Adams and Pasternack [ 301 also observed vibrational excitation of NH( a ‘A, Y=O-2) in the same (HNJ 266 nm) photolysis. Moreover, Hack and Mill [ 91, using the LIF technique, have recently shown that the overall rate constants for the reaction NH( a ‘A, v= O-2) + HN3 have little dependence on the vibrational levels of NH( a ‘A). In the present study, the rate constant k3 measured at different NH2 wavelengths are close to each other, which is consistent with the results obtained by the LIF technique. All findings in the present experiment give rise to the conclusions that the visible emission originates from vibronically excited NH*, that all vibrational levels of NH2 (A *A,) are produced in the reaction of NH( a ‘A) with HNs, and that highly vibrationally excited NH2 (A *A1) is produced from vibrationally excited NH(a ‘A, u). 109
Volume 204, number 1,2
CHEMJCAL PHYSICS LETTERS
Acknowledgement
The authors are indebted to Okitsugu Kajimoto (Kyoto University), Tomoko Kimura and Aki Tanaka (Niigata University) for their valuable help to perform the experiments.
References [ 1 ] J.R. McDonald, R.G. Miller and A.P. Baronavski, Chem. Phys. Letters 51 (1977) 57. [2] A.P. Baronavski, R.G. Miller and J.R. McDonald, Chem. Phys. 30 (1978) 119. [3] J.R. McDonald, R.G. Miller and A.P. Baronavski, Chem. Phys. 30 (1978) 133. [4] LG. Piper, R.H. Krech and R.L. Taylor, J. Chem. Phys. 73 (1980) 791. [5] F. Rohrer and F. Stuhl, Chem. Phys. Letters 111(1984) 234. [6] F. Freitag, F. Rohrer and F. Stuhl, J. Phys. Chem. 93 (1989) 3170. [7] J.W. Cox, H.H. Nelson and J.R. McDonald, Chem. Phys. 96 (1985) 175. [ 81 W. Hack and A. Wilms, Z. Physik. Chem. 161 ( 1989) 107. [9] W. Hack and Th. Mill, J. Phys. Chem. 95 (1991) 4712. [lo] J.R. McDonald,J.W. Rabalaisand S.P. McGlynn, J. Chem. Phys. 52 (1970) 1332. [ I1 ] K. Dressler and D.A. Ramsay, Phil. Trans. Roy. Sot. (London)A251 (1959) 553. I 121S.C. Ross and F.W. Birss, M. Vervloet and D.A. Ramsay, J. Mol. Spectry. 129 (1988) 436.
110
I2 March 1993
[ 131F.W. Birss, M.-F. Merienne-Lafore, D.A. Ramsay and M. Vervloet, J. Mol. Spectry. 85 ( 1981) 493. [ 141D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Chumey and R.L. Nuttall, J. Phys. Chem. Ref. Data 11 Suppl. 2 (1982). [ 151 W.R. Anderson, J. Phys. Chem. 93 (1989) 530. [ 161K.P. Huber and G. Herzberg, Molecular spectra and molecular structure, Vol: 4. Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1979). [ 171 G. Herzberg, Molecular spectra and molecular structure, Vol. 3. Electronic spectra and electronic structure of polyatomic molecules (Van Nostrand Reinhold, New York, 1966). [ 181T.C. Clark and M.A.A. Clyne, Trans. Faraday Sot. 66 (1970) 877. [ 191 M.J. Pellerite, R.L. Jackson and J.I. Brauman, J. Phys. Chem. 85 (1981) 1624. [201 J. Habdas, S. Wategaonkar and D.W. Setser, J. Phys. Chem. 91 (1987) 451. [21] J.M.L.MartinandJ.P. Fraqois, J.Chem.Phys.93 (1990) 4485. [ 2210. Kajimoto, T. Kawajiri and T. Fueno, Chem. Phys. Letters 76 (1980) 315. 1231B.M. Dekoven and A.P. Baronavski, Chem. Phys. Letters 86 (1982) 392. [24] F. Rohrer and F. Stuhl, J. Chem. Phys. 88 (1988) 4788. [ 251 K.-H. Gericke, M. Lock and F.J. Comes, Chem. Phys. Letters 186 (1991) 427. [ 261 J.B. Halpem, G. Hancock, M. Lenzi and K.H. Welge, J. Chem. Phys. 63 (1975) 4808. [27] LJ. Wysong, J.B. Jeffries and D.R. Crosley, J. Chem. Phys. 93 (1990) 237. [28] G. Le Bras and J. Combourieu, Intern. J. Chem. Kinetics 559 (1973) 559. [29] H.H. Nelson and J.R. McDonald, J. Chem. Phys. 93 (1990) 8717. [ 301 J.S. Adams and L. Pastemack, J. Phys. Chem. 9 5 (1991) 2975.