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Detection and quenching of NH(b 1Σ+) in the pulsed vacuum UV photolysis of NH3
Vo!ume 33, number 2
CEDXlCAL PHYSICSL5TERS
I fune f97.5
DETECTTON AND QUENCRHNG 0F NFf(b ‘C’) IN THE PULSED V_4CUUM U-VPKOTOLYSIS OF W3 C. ZETZSCH and F. STUHL
Received 21 Februxy
frcn
1975
The ‘kinetics of NH(b ‘Z+ ) wx stcdied in the pulsed vxuum UV photolysis of NH; ustig the m~issioi~ at 4707. 1 X the forbidden trmsition NH(b ’r+ - ,K3Z-). Abso!utc rate canstm~s were determined for the quenching by NH3
1. Introduction
Recently Gilles et al. [l] have reported a new emission at 47!0.1 * 2.5 A in the vacuum UV photolysis of NH,. They attributed this emission to the Q-branches of the forbidden transition NkI(b ‘C’ ; u’= 0 + X 3C- ; V ” = 0) 2nd they determined the energy difference between the singlet and triplet system of NH. In the pulsed vacuum UV photolysis of NH3 we also observe an emission at approximately the same wavelength which lasts up to severa! ms. In this note we would like to report the first measurements of t!~e lifetime of this emission in the presence of NH; and AT.
(max. transmission of 50% at 4707 A: half bandwidth 7 .k) by a photomultiplier (EM type 9789 QE). The time resolved signal (resolution up to 1Ci
to be ti20.
N2, and 0,.
3. Resuits and discussiorr 2. Experimental was The apparatus used in the present experiments si_milar to that described previously in kinetic studies on m&stable species [2] and on radicals [3 J. Briefly, the apparatus consisted of a pulsed vacuum ‘UV light source, a reaction chamber and a detection system for
the NH(b 1 X:’+ X 3 C-) emission. NH(b IX’) radicals were produced in the pulsed vacuum UV photo!ysis (LiF window; X > 1050 ii) of NH; _Pulses of vacuum W light were generated by discharging a capecitor (20 j) with a repeiiiicn frequency of about 1.5 Hz. The NH emission was detected at right angles to the incident ligllt pulses through an interference filter
In order to determine the wavelength of masimum transmission of the interference filter io be used, the spectrum of the forbidden NH emission was reinvestigated. For this purpose a discharge flow system in which NH, was added downstream to XI Ar disclxrgs was used to generate NH(b IC’) radicals. ‘Their emission to the ground state was obsewed with a rcso!ution of abaut 0.2 _&using a 0.5 m grating monochromator (Minuteman, type 305 X). The spectroscopic data obtained from ihis speLtrum 2~ suLmnarizec! a~12 compared with literature values in tab!e 1. .4n emission at 4705 A w2s also generated by flash photolysing NH, at pressures from 10W3 to 0.7 torr. (In the photolysis of NH3 it is eneigeticaliy possible to
VoIume 33, number 2 Tl,ble I Compsri~on oispectroscopic mnsition of NH
i&.
[S]
Bb (cm-’ )
a;; (cm-l)
16.430
16.345
d2ta
of the (b ‘Et
10000
i X3X-)
I
I
:
10
I
uoo (cm-‘)
ref. [l]
15.9
15.9
21231k11
this work
16.3
16.3
21X&
(4710.1
i-5 (4707.1
A) A)
form NH(b IYj at wavelengths below 1860 _?I.) TiltLangthe interference fItt:r resulted in a marked decrease of the transmitted light which is taken as evidence for an einission at 4707 .4 in agreement with the spectroscopic results. Since this emission decays s!owly (7 < 2.5 ms) in the present experiments it is very likely that it originates from the metastable NH (b I,+) radicals. It is kn~\v~~th~tthe V~CUUITI LiV photolysis of NfIj also yields excited NH(c ‘II) and excited NH2(x “A:) [4]. Emissions from these states might interfere in the present experiments. However, the radiative !ifetime of NH(c 1 II) is too short (7 z 4X 10m7 s) [5] to contribute to the observed emission at 4707.1 A Furthermore, the radiative lifetime of NHz(x 2A1 j has been previous!); suggested to ‘3e S X 10U6 s 1_6], In :he present system the actual lifetiie of NH2(A l/l,) will be even shorter skce this state is quenched by NH, in almost every coilision [6]. Thus, emissions from excited NH, arr u_nI&e!y to interfere with the !ong lasting emission at 4,707 .I W. It was observed in ,2! pulsed photolysis experiments th_t the NM(b ‘C’) radicals decay exponentially. As an example fig. 1 shows a sernilogarithmic plot of the NH@ lx:‘) concentration as a function of time. It should be noted that linearity holds in this plot for more th;m two decades of decreases in NH(b lIC+) concentration. From decays like these, lifetimes, 7: of the met&able NH and deca!r rates, r-: ? were determined a~ a function of the concenirsiion of NH, and Ar. This, in turn, permits us to determine rat@ constar?ts for the quenching of NH:5 ‘Zi) by NH3 and Ar as ShCWiIl beiow.
From the estimated li:$t output of the vacuum UT flash lamp and from tile .zbsorption of NH3 approximate upper limits {or the concentrations of NH(b lx’) were estimated to bz 5 X lOI cm-j for iO-? torr for 0.5 torr _?JH3 _Thus, i33, and to be 3 X 1O1; m-3 376
z Sum 197.5
CHEMICAL PHYSICS LATTERS
I
I
Tine
(in
t&r&x
d Chmnsls
I
i5
1
Fii. ‘_ Time dependence of the intemity of the NH(b *Xi X s : mnm~on 2.t 4707 A for 0.1, 0.2,0.4 and 0.6 rorr NH3. The time scale is given by tie chmnel numbers of the multichannel mzlyzcr. The width of a chum! is 100 PS.
NH3 and Ar are a!ways Fresent in this system in large excess over the concentration of NH(b lZ+j and the present data can be analyses! using the equation
7-l =~{~;+/c~[NHJ]
+Fcz[Ar].
In this equation ro is the lifetime ofNH(_o lx’) in the absence of NH3 and of Ar. This lifetime includes the radiative lifetime, quenching by possible impurities inherent to the reaction chamber, and diffusion effects. The rate constants of the reactions
INH@ 12’) -I-NH, + products, iW(b
lx+)
I Ar + products
(1) (2)
, respective!y. Plots of 7-l verze given by k, md X-_, sus [h%,;] and versus [Ar] are shown in fig. 2. Inc!uded in this $ure are four data points for various pressures of NH3 in the presence of 10 torr Ar. These daza show that the !ar gz decay rates observed at low pressakes of NH3 (dashed JLneIn fig. 2) 2s due to diffusion.
Volume 33, zvxnb+r 2
CREMlCAL
1 func 195.5
PHYSICS LE-ITERS
the preszr).f: WA. HOWreaction (2) describes a physical! quenching process. In addition io physic31 quenching the forixtian of 2 M-i2 should he considerid cannOt be inferred from suer, ig caz be assumed rhat
ed in rezctian
(1).
Impurities contained ia the present sarnp!~s sho~lci only play a mirlor role in the quenching of NH(b II?). Since NH, is strong& adsorbed cm the wails of the rcriction chamber, the pressure measurements of MI%; are estimated to be accurate within i 20% on&. The we&f! accusacy for k, and k2 is estimated to be * 25%. Rate co?lsQants for the quenchln2, of NH(b W) 2x not aailable in the literature. ~o~~~ri~g ?he pxsent vatues ofk, and I;, with those for tk relevant o.xy_een species it appears that the quenching: behavior of NIi (b lZ?> is closer io that ofO,(b ‘C”) rather than of 0@4), O(%) or O(lD). -
Acknowledgemenh
References A. Giiies, 5,
Since these four data goints represent the quenching by both NH, 3nd 10 torr Ar, they correspondingly lie rrbove the straight line drawn for pure NX3. Ti~!edopes of the sirai&t Iines in fig. 2 determine k, and k, to be 4.1 X IO-13 and 3.5 X 1W16 cm3 m~lecule=~ s-l> respectively. Froin &tieintercepts in Eg. 2 the radiative Efetime of NH@ i Z?) is determined to be 3 5 ms. The quenching mechanism, whether physical or chcm-
.tfa~~!nctand C. V
25 (1974) 346;
J. hkssanet,A. Cilles znd C. Velmeii, J. Fhotochem. 3, to be pub!ished. E-‘.Stuhlttnd I-f, Niki, Chem. Phys. Letters 7 (197Oj 473. F. Stllhl, BX. BUnSengCS. Phpik. Chem. 78 (1974pi) 230, H. Okzbe ;?sd lif. Lenri, J. &hem. Phys, 47 (1967) 5X. J.X. Lents, I. Quant. Spectry. Radiatise Tmnsf% 13 (5473) 397. !A Len& J.R. KcNesby, A. lifele 2nd C. Ngzyea Xuan, J. Chum. Phys. 57 6f972) 319.
CEDXlCAL PHYSICSL5TERS
I fune f97.5
DETECTTON AND QUENCRHNG 0F NFf(b ‘C’) IN THE PULSED V_4CUUM U-VPKOTOLYSIS OF W3 C. ZETZSCH and F. STUHL
Received 21 Februxy
frcn
1975
The ‘kinetics of NH(b ‘Z+ ) wx stcdied in the pulsed vxuum UV photolysis of NH; ustig the m~issioi~ at 4707. 1 X the forbidden trmsition NH(b ’r+ - ,K3Z-). Abso!utc rate canstm~s were determined for the quenching by NH3
1. Introduction
Recently Gilles et al. [l] have reported a new emission at 47!0.1 * 2.5 A in the vacuum UV photolysis of NH,. They attributed this emission to the Q-branches of the forbidden transition NkI(b ‘C’ ; u’= 0 + X 3C- ; V ” = 0) 2nd they determined the energy difference between the singlet and triplet system of NH. In the pulsed vacuum UV photolysis of NH3 we also observe an emission at approximately the same wavelength which lasts up to severa! ms. In this note we would like to report the first measurements of t!~e lifetime of this emission in the presence of NH; and AT.
(max. transmission of 50% at 4707 A: half bandwidth 7 .k) by a photomultiplier (EM type 9789 QE). The time resolved signal (resolution up to 1Ci
to be ti20.
N2, and 0,.
3. Resuits and discussiorr 2. Experimental was The apparatus used in the present experiments si_milar to that described previously in kinetic studies on m&stable species [2] and on radicals [3 J. Briefly, the apparatus consisted of a pulsed vacuum ‘UV light source, a reaction chamber and a detection system for
the NH(b 1 X:’+ X 3 C-) emission. NH(b IX’) radicals were produced in the pulsed vacuum UV photo!ysis (LiF window; X > 1050 ii) of NH; _Pulses of vacuum W light were generated by discharging a capecitor (20 j) with a repeiiiicn frequency of about 1.5 Hz. The NH emission was detected at right angles to the incident ligllt pulses through an interference filter
In order to determine the wavelength of masimum transmission of the interference filter io be used, the spectrum of the forbidden NH emission was reinvestigated. For this purpose a discharge flow system in which NH, was added downstream to XI Ar disclxrgs was used to generate NH(b IC’) radicals. ‘Their emission to the ground state was obsewed with a rcso!ution of abaut 0.2 _&using a 0.5 m grating monochromator (Minuteman, type 305 X). The spectroscopic data obtained from ihis speLtrum 2~ suLmnarizec! a~12 compared with literature values in tab!e 1. .4n emission at 4705 A w2s also generated by flash photolysing NH, at pressures from 10W3 to 0.7 torr. (In the photolysis of NH3 it is eneigeticaliy possible to
VoIume 33, number 2 Tl,ble I Compsri~on oispectroscopic mnsition of NH
i&.
[S]
Bb (cm-’ )
a;; (cm-l)
16.430
16.345
d2ta
of the (b ‘Et
10000
i X3X-)
I
I
:
10
I
uoo (cm-‘)
ref. [l]
15.9
15.9
21231k11
this work
16.3
16.3
21X&
(4710.1
i-5 (4707.1
A) A)
form NH(b IYj at wavelengths below 1860 _?I.) TiltLangthe interference fItt:r resulted in a marked decrease of the transmitted light which is taken as evidence for an einission at 4707 .4 in agreement with the spectroscopic results. Since this emission decays s!owly (7 < 2.5 ms) in the present experiments it is very likely that it originates from the metastable NH (b I,+) radicals. It is kn~\v~~th~tthe V~CUUITI LiV photolysis of NfIj also yields excited NH(c ‘II) and excited NH2(x “A:) [4]. Emissions from these states might interfere in the present experiments. However, the radiative !ifetime of NH(c 1 II) is too short (7 z 4X 10m7 s) [5] to contribute to the observed emission at 4707.1 A Furthermore, the radiative lifetime of NHz(x 2A1 j has been previous!); suggested to ‘3e S X 10U6 s 1_6], In :he present system the actual lifetiie of NH2(A l/l,) will be even shorter skce this state is quenched by NH, in almost every coilision [6]. Thus, emissions from excited NH, arr u_nI&e!y to interfere with the !ong lasting emission at 4,707 .I W. It was observed in ,2! pulsed photolysis experiments th_t the NM(b ‘C’) radicals decay exponentially. As an example fig. 1 shows a sernilogarithmic plot of the NH@ lx:‘) concentration as a function of time. It should be noted that linearity holds in this plot for more th;m two decades of decreases in NH(b lIC+) concentration. From decays like these, lifetimes, 7: of the met&able NH and deca!r rates, r-: ? were determined a~ a function of the concenirsiion of NH, and Ar. This, in turn, permits us to determine rat@ constar?ts for the quenching of NH:5 ‘Zi) by NH3 and Ar as ShCWiIl beiow.
From the estimated li:$t output of the vacuum UT flash lamp and from tile .zbsorption of NH3 approximate upper limits {or the concentrations of NH(b lx’) were estimated to bz 5 X lOI cm-j for iO-? torr for 0.5 torr _?JH3 _Thus, i33, and to be 3 X 1O1; m-3 376
z Sum 197.5
CHEMICAL PHYSICS LATTERS
I
I
Tine
(in
t&r&x
d Chmnsls
I
i5
1
Fii. ‘_ Time dependence of the intemity of the NH(b *Xi X s : mnm~on 2.t 4707 A for 0.1, 0.2,0.4 and 0.6 rorr NH3. The time scale is given by tie chmnel numbers of the multichannel mzlyzcr. The width of a chum! is 100 PS.
NH3 and Ar are a!ways Fresent in this system in large excess over the concentration of NH(b lZ+j and the present data can be analyses! using the equation
7-l =~{~;+/c~[NHJ]
+Fcz[Ar].
In this equation ro is the lifetime ofNH(_o lx’) in the absence of NH3 and of Ar. This lifetime includes the radiative lifetime, quenching by possible impurities inherent to the reaction chamber, and diffusion effects. The rate constants of the reactions
INH@ 12’) -I-NH, + products, iW(b
lx+)
I Ar + products
(1) (2)
, respective!y. Plots of 7-l verze given by k, md X-_, sus [h%,;] and versus [Ar] are shown in fig. 2. Inc!uded in this $ure are four data points for various pressures of NH3 in the presence of 10 torr Ar. These daza show that the !ar gz decay rates observed at low pressakes of NH3 (dashed JLneIn fig. 2) 2s due to diffusion.
Volume 33, zvxnb+r 2
CREMlCAL
1 func 195.5
PHYSICS LE-ITERS
the preszr).f: WA. HOWreaction (2) describes a physical! quenching process. In addition io physic31 quenching the forixtian of 2 M-i2 should he considerid cannOt be inferred from suer, ig caz be assumed rhat
ed in rezctian
(1).
Impurities contained ia the present sarnp!~s sho~lci only play a mirlor role in the quenching of NH(b II?). Since NH, is strong& adsorbed cm the wails of the rcriction chamber, the pressure measurements of MI%; are estimated to be accurate within i 20% on&. The we&f! accusacy for k, and k2 is estimated to be * 25%. Rate co?lsQants for the quenchln2, of NH(b W) 2x not aailable in the literature. ~o~~~ri~g ?he pxsent vatues ofk, and I;, with those for tk relevant o.xy_een species it appears that the quenching: behavior of NIi (b lZ?> is closer io that ofO,(b ‘C”) rather than of 0@4), O(%) or O(lD). -
Acknowledgemenh
References A. Giiies, 5,
Since these four data goints represent the quenching by both NH, 3nd 10 torr Ar, they correspondingly lie rrbove the straight line drawn for pure NX3. Ti~!edopes of the sirai&t Iines in fig. 2 determine k, and k, to be 4.1 X IO-13 and 3.5 X 1W16 cm3 m~lecule=~ s-l> respectively. Froin &tieintercepts in Eg. 2 the radiative Efetime of NH@ i Z?) is determined to be 3 5 ms. The quenching mechanism, whether physical or chcm-
.tfa~~!nctand C. V
25 (1974) 346;
J. hkssanet,A. Cilles znd C. Velmeii, J. Fhotochem. 3, to be pub!ished. E-‘.Stuhlttnd I-f, Niki, Chem. Phys. Letters 7 (197Oj 473. F. Stllhl, BX. BUnSengCS. Phpik. Chem. 78 (1974pi) 230, H. Okzbe ;?sd lif. Lenri, J. &hem. Phys, 47 (1967) 5X. J.X. Lents, I. Quant. Spectry. Radiatise Tmnsf% 13 (5473) 397. !A Len& J.R. KcNesby, A. lifele 2nd C. Ngzyea Xuan, J. Chum. Phys. 57 6f972) 319.