Mercury-photosensitized luminescence of NH3 and ND3 at low pressure

Volume 5 2, number 1

MERCURY-PHOTOSENSITIZED

T. HIKIDA, Department

15 November

CHEMICAL PHYSICS LETTERS

T. ZSIIIHARA of Chemistry.

LUMINESCENCE

1977

OF NH3 AND ND3 AT LOW PRESSURE

and Y. MORI

Tokyo Institute

of Techrrology,

Tokyo, Japan

Received 4 July 1977 Revised manuscript received 16 August 1977

The mercury-photosensmzed with

[ND3

] = 0.005

luminasccncc

of

.unmoni~ hzls been investigated

at low pressures. The emission spectrum

torr consisted of two peaks at 305 and 340 run. The 305 nm band WBS.lssigned to the complcv in the

higher electronic state which correlates with Hg(3Pt). At limiting zero pressure, the complex formation in the hi&r state predominates, whdo the 340 nm hand is in the ascendant at higher pressure. Results obtained m the prcscncc of third-bodies, M = CzI11;, CH4, CF4, N2, and Ne hzve been also reported.

1. Introduction The mercury-photosensitized reaction of ammonia has been known to induce a rather strong emission in the ultraviolet region. General aspects of the emission have been well understood in terms of the mercuryammonia complex arising from bi- and tcrmolecular reacticns of Hg(sP,) and ammonia. Reaction rate constants for both bi- and termolecular processes have been measured with a phase shift technique [ 11, and with time resolved spectroscopy [2]. Callcar

aild Connor

[3J huvc investigated

the crnis.

sion spectra at 0.3 torr NH3 which extended down to 265 nm; its maximum intensity at 344 nm has been assigned to the emission from the unstabilized complex

formed

in bimolecular

collisions.

Their

COII-

elusion entirely depends on the reported rate con. stants. Discussion on the emission profile, however, may not be conclusive since the potential energy curves for both the ground and the excited states are not known. Recently we have investigated [4J the emission spectra with NH, pressures as low as 0.01 torr. It was found that at NH, pressure lower than 2 torr, a broad emission band with its maximum intensity at ~305 nm became significant, in addition to the 340 nm emission arising from the stabilhed complex. It has been shown that the observed spectral changes at low pressures are not in accord with the reported reaction

rate constants of bi- and termolecular processes. The 305 nm emission has been tentatively assigned to the transition from a higher excited state which correlates \vlth Hg(31’,). Tills upper state rnny be formed eltber by direct collisions of Hg(3Pt) and ammoma or by an inverted predissoclation from the unstabilired Hg(3Po)NH, complex. More recently Callear and Freeman [S] reinvestigated the time resolved luminescence from the HgNH, COIIIplex at low pressures. They concluded that the carrier of the 305 nm emission is an unstabilized complex HgNH3 formed in a bimolecular encounter of H&PO) with NH,. The 305 nm emission has been ascrlbed to occur by way of an inverted prcdissoclation, crossmg over from the A, state which is formed in the encounter of IIg(3Po) with NH,, to a state which correlates to Hg(3P1). A devaluation of the bmlolecular rate constant by a factor of 3, together with a small emission quantum yield (=O.l), has been suggested. This paper is to report an investigation on the rclative intensity variation of the 305 and the 340 nm emission with NH,, and with ND3 pressures. Similar results obtained in the presence of third-bodies, such as C2H6, ci-i4, CF,, N2, and Ne ilrc also reported.

2. Experimental

The apparatus

and the experimental

procedure

are 43

,

ClIlX~ICAL

Volurrw 52, number 1

the same as those described previously [4]. The reaction cell was made of fuzed silica with a volume of ~300 cm3 and its wait; were coated by a tetrafluoroethylene polymer. All the samples used in this experiment were purchased commercially and are handled as described previously. The sample pressures were measured by a Plrani gauge calibrated by a McLcod gauge when the pressures were low and by a manometer for higbcr pressures.

3. Results and discussion The profilcs of the emission bdnds obtained with ND, are shown m fig. 1. Curve (a) of fig. 1 IS a band emission obtained for ND, at 30 torr. This spectrum is identical to those reported by Phillips et al. [6], and also is similar to that observed with NH3 [4]. The emission given by the dcuterated molecule is slq$tly red-shifted. These spectra have been assigned to the stabilized complex arising from a reaction of Hg(3P,) with ammonia in termolccular collisions [3]. Curve (b) of fig. 1 is the emission spectrum obscrvcd with ND3 at 0.005 torr. Some cmisslon lines from the excitation lamp are superimposed. Similar emission profiles arising from the reaction of NH, have been reported for NH3 at 0.08 torr [4]. The emission spectrum given by the ND, complex is much more intense than that from the NH, reaction when the pressure of ammonia is low. This seems to be consistent with

Wavelength (nm) keg. 1. Emission spectra of I&ND3 30.0torr; (b) [ND31= 0.005torr.

complex: (a) [ND31=

PIIYSICS LITTERS

15 November

1977

the fact that ND3 is much more stable than NH3 In rcspcct

to the mercury-photosensitized

decomposition

171. We have shown that the emission profiles of the HgNH3 CCJJtl~kX at hw pressures may be explained in terms of superposition of two emission band profiles [4] ; the spectrum of the stabitizcd complex with i:s maximum intensity at 340 nm and another band with its maximum

at 305

nm. The spectral

profiles

of

the

deutcrated complex shown in fig. 1 are again analyzed by the same procedure, that is, curve (b) is assumed to be a superposition of two emission profiles, one of them being curve (a). Subtracting curve (a) from curve (b) after normali&ation of the peak height at 340 nm, the residual spectlnl profile thus obtained is found to bc close to the reported emission band of the HgNH; complex with its maximum at 305 nm. Thus the mercury-photosensitized emission of ND, seems also to consist of the emission carriers in two different excited states, though the spectrum is slightly red-shifted. The accuracy in the procedure of subtraction is much higher in case of ND, since the cmisSIOI~ mtensity of the complex is considerably higher wlvhen the pressure is !ow. The emission intensities at 305 and 340 nm were obtained after the subtraction mentioned above. it was found that the intensities at both 305 nm and 340 nm increased with the prcssurc of ammonia, but we failed to obtain a clear relationship, probably bccause of the imprisonment effect and the pressure broadening effect. Only the ratio of intensities, 1340/1305, was calculated for various pressures of NH3, and of ND3. Thcsc are shown in fig. 2. For pressures of NH3 between 0.01 and 1.2 torr and of ND3 between 0.005 and 0.4 torr, Z340/1305 iricrcascs linearly with pressure, with slopes of 6.24 and 5.83 torr-l, respectively. The intercepts slightly exceed unity for both NI13 and ND3. These results arc satisfactorily explained by the biand termolecu!ar reaction mechanism which results in two different excited states. The 340 nm band has been assigned to the emission due to the stabilized complex between ammonia and Hs(~P~). An accumulation of evidence has been reported to support this assignment [ 1,2]. The 305 nm entission band has a profile similar to the 340 nm band but is shifted to shorter wavelengths. The emission spectra from higher vibrational levels of electronically excited polyatomic

44

Volume 52, number

8-

s

3

0

‘0

1

CHI:MICAL

I’tIYSICS LLTTEIIS

15 November

197 /

(a)

I

I

I

02

0

04

06

I

I

10

12

I

08

i-3

:;::.--:1

01

0

02

03

molecules often appear at wavelengths close to those from lower vibrational lcvcls. The carrier of the 305 run cmtssion band, therefore, should be assigned to the complex excited to a higher excited state thnn the 340 nm band, though the potential surface of the complex is not known completely. These higher exerted

Hg(3P,)

states of the complex probably

correlate

to

that 1340/f305 increased linearly C2116, CI14, CF4, N2, and Ne, results are shown in fig. 3 for

for ND, (0.005 torr) with Ne as third-body. The intercepts of all the linear plots are found to be constant at 1.2 2 0.1, while the slopes differ significantly; 6.24 torr-l for NH3 to 0.0799 torr-t for NC. These are summarized in table 1. The results shown m table 1 seem to be quite consistent Hs(3Po)

with the bi- and termolccutar by NH,,

mechanism Hg(3Pt)

+

quenching

of

and by ND,. For NH, the reactron

[SJ is

NH, +Hg(3Po)+NH, ,

Hs(~P~) + NII, +pHgNH; Hg(3P,)

‘I-able 1

Slopes of plots, 134a/13es vcrsu prcssurc --_-_----

+ (I-

P)HgNH3

f NH, + M + HgNH3 + M ,

(1) ,

(2) (3)

HgNH; + Hg + NH3 + 305 nm emission ,

(4)

HgNH, + Hg + NH, + 340 nm emission ,

(5)

-- ----.---_-.-

NH3 (lox-1 ) - --_ - --

ammonia %I’6

c114

[5,61-

It was also f’oiinci with the prcssurcs of as third-body. Some NH3 (0.01 torr) and

16

Trg. 3. Ratio of t3rewnauon intcnslty at 340 and 305 nm vcrsw pressure of NC: (,I) (NH31 = 0.01 tom; (b) [ND31 : 0 00.5 torr.

at 340 and 305 nm

versus pressure of (a) Ntls, and (b) ND3.

electronic

12

8

CNel torr

Ammonla (tow) big. 2. Ratio of the emission mtcnslty

4

0

04

cl-q NZ Nl!

--_NI)1

(torr-1 ) --

-.---------

6.24 2.42 0.734 0.425 0.369 0 0799 _____--_--

HgNI I; + products

_

5.83 -

----

0541 0.374 0.101 --

-----

(6)

Here, the 305 nm emission results from the I IgNI-1; complex formed by reaction (2). This cornplnx IS also subJected to prcdissociation which leads to the decomposition of ammonia. The stabrltZed complex, 1IgNH3 formed by reaction (2) and also by reactton (3) has its intensity maxm~um at 340 nm. At lmiiting lero pressure, the cxtrapol~ted value of 1340/1305 and the emission quantum yield (~0.1) of HgNIl; [5], seem to indicate that reaction (2) is much 111favor to the HgNH; with p x 0.9. The ratio, k3/k2, can be computed from the slope and the mtcrcept of a particular kind of M, provided that the emission quantum yield of HgNII; is known. A value of 1.7 X 10--L7 cm3/ molecule is obtained for M = NH3 by assuming a quantum yield of 0.1. This may be compared to 45

Volume 52, number 1

CHEMICAL

PHYSICS

0.59 x lo-l7 cm3/molccule, calculated from the reported reaction rate constants. A devaluation of the bimoIccuIar rate constant (kk2) by a fiictor of 3 has been suggcstcd j.51 and if this is taken into account the agreement is even better. Similar relationship may be obtained for other third-bodies listed in table 1. However, uncertainties on the value of k, and of the emission quantum yields, particularly for ND, make further comparrson Gf k3/k, less mei~ningful. It is intercstmg to note that the relative values of the slopes Irsted in table 1 :lrC closely related to the reported termolccular rcactlon rate constants or various molccules [I $1. The value ofp, which wds indiLated to be 0.9 for NH,, should be understood as the extent of the Hg(3Pd) consumption by reactions (4) and (6), and not as the probability of the formation of HgNH;, the unstabilized complex by encounter collisions of Hg(3P0) and NH3 [reaction (2)J. The inverse p~cdissociation forming HgNI 1s and HgNH1(u) is expected to have a much hrgher probablIity for IIgNHj(u) formation thap for HgNH; formation, that is the bimolecular encounter of Hg(sP,) and Nil, mostly rcsuits in the formation of the complex in tfle lower electronic level which correlates to Ilg(3P0) with excess vibrational energy. The

cornplcx

formed

by the bimolecular

Hg(3P0) + NH, + XHgNH; + (1 - h)HgNH+)

HgNH#)

0%

+M,

+ M 9 HgNH, + M

, (2’) (7)

--f Hg(3Pg) + NH, ,

HgNH; +M+HgNI13 HgNH,(u)

f NH, ,

(9) ,

(10)

h is the probablhty of the formation of HgNH; in reaction (2’) and ~111have D small value. The complex with excess vibrational energy HgNH,(u) can

where

46

also emit the 340 nm band [reaction (S)] _However there may be alternative explanations of reactions (2), and (3), for instance, Hg(3 I’,,) + NH, + HgNH3(u) ,

(11)

HgNH3(u) d Hg(sP,)

(12)

+ NH, ,

HgNH3(u) + H&NH; ,

(13)

HgNH; + HgNH3(u),

(14)

HgNH; -f M --f IlgNI13 + M , HgNIf3(u) -f-M --f HgNH3 t M .

(9)

(IO)

In this case, the probnbility of formation of HgNHz is not required since the rate constants k12, and k13, dominantly determine the population ratio between HgNH; and HgNH3(u) at low pressure. Nevertheless, the experimental observation may be explained in terms of the simple bi- and termolecular reaction mechanism which has been interpreted from the purely phenomenological mechanism obtained under high NH, pressures. The reactions with low ammonia pressures arc undoubtedly far more complex. Further experimental investigations and theoretical interpretation are urgently needed to elucidate the mechanism under low ammonia pressures.

collision

of Hg(sP,) and NH, canriot have a long lifetime without the collisional deactivation by third body. The complex has enough energy to dissociate into Hg(sP,) and NH,. This back reaction does not consurnc Hg(sP,,) and is sccmingIy faster than reactions consump(4), (5) and (6); tl ie c-11il~lIlClS Of IIg(3Pg) tion. Thus, reactions (2) and (3) may be replaced by the following reaction scheme:

HgNH; + Hg(3P,)

15 November 1977

LETTERS

References C.G. Freeman, M.J. McEwan, R.F.C. Claridgc and LX. Phillips. Chcm. Phys. Letters 9 (1971) 578; Trans. P.uaddy Sot. 67 (1971) 2004. I21 A-1) Callear and J.C. McGurk, Chcm. Phys. Letters 7 (1970) 491; J. Chem. SW. Faraday II 69 (1973) 97; A.B. Callear, J.H. Connor and J. Koskikdlio, J. Chem. Sot. I-iuaday II 70 (1974) 1542. [31 A.B. Callear and J.11. Connor, J. Chem. Sot. Faraday II 70 (1974) 1667. I41 T. Hlkida, T. lchimura and Y. Man, Chcm. Phys. Letters 27 (1974) 548. ISI A.B. Callear and C.G. Freeman, Chern. Phys. Letters 45 (1977) 204. hf.J. McEwan, R.F.C. (61 R.H. Newman, C.G. rrceman, Claridge and L.F. Phillips, Trim>. Faraday Sot. 66 (1970) 2627. 171 Y. Mori, H. Kadoi and I. Tanaka, Nippon Kagaku Zasshi 86 (1965) IG3.

[II