- Email: [email protected]
The excited mercury—ammonia emission at low pressure
CHElMICAL PHYSICS LETTERS
Volume 45, number 2
THE EXCITED MERCURY-AMMONIA Anthony
15 January 1977
EMISSION AT LOW PRESSURE
and Colin G. FREEMAN*
B. CALLEAR
Physical Chemistry Department. Carnbndgc CB2 ZiP. UK
University
of Cambridge,
Received 4 October I976
The luminescence from Hg, NH3 mixtures, when excited with 2537 A resonance radiation, has been investigated at low pressures (> 0.03 torr) with a time resolved technique. It is concluded that the carrier of the band with a maximum at = 3000 A is an I&NH: complex formed in a bimolecular encounter of Hg(6 3Po) with NH3. The quantum yield of the bimolecular emission is small (O.l), the main channel for bimolecular removal of Hg(6 3Po) being radiaticnless.
1_ Introduction
HgNH; + Hg + NH, + hv _
When mixtures are ejrcited-with
let luminescence an intensity
OF ammonia
acd mercury
vapour
2537 a resonance radiation, ultraviooccurs via a broad diffuse band, with
maximum
at about
3500
A.
For the most part, it appears that the emissicn is associated with the interaction of Hg(6 3Po) (hereafter Hgo) with NH, _With a phase shift technique [ 1,2], it has been established that the emission is delayed relative to the excitation at 2537 A; Hg(6 3P,), formed initially, undergoes spin orbit relaxation to Hgo which is removed slowly at low pressure to generate the delayed fltiorescence Hg(63P1)+NH3+Hg0+NH3,
k,
Hgo &NH3 :
Hg+NH3
(1) +hv.
(2)
Relaxation times of Hgo and decay constants of the ultraviolet emission were found to be indistinguishable with a flash photolysis method [3], fcr 7 2 20 ps; from an analysis of the effect of inert gases it was established that removal of HgO by NH3 also occurs in termolecuIar co&ions: Hgo +NH,
+M
-+HgNH;
+M,
(3)
* On leave from the University of Canterbury, Christchurch, New Zealand.
204
(4)
With high pressures of NH,, the luminescence lags behind the Hgo decay profile because of the finite radiative lifetime of the stabilised HgNH; complex of = 1.8 J.S. Rate coefficients derived from phase shifts [ 1,2], flash photolysis experiments [4], and phase sensitive modulation spectroscopy ES,6 1, are in good agreement. The bimolecular coefficient k2 has been reported to be 3.2 (kO.3) X IO-l3 cm3 molecule-’ s-l and the termolecular coefficient k, (M = NH$2.2 (20.3) X 10m30 cm6 molecuIe-3 s-l at about 293 K. The spectral profile of the luminescence varies with pressure of NH, and also with pressure of added inert gas. In this letter we are not concerned with effects zt high pressures of ammonia where Hgo may attach clusters of NH, [7] _ The emission profile with NH3 at about 1 torr, and with an atmosphere of added N is 2,’ almost entire!y due to stabilised monomer, HgNH,. This spectrum is illustrated by curve a of fig. 1. To examine the emission from HgNH; formed via reaction (2), Callear and Connor [7], using cw excitation, recorded the spectrum with NH3 at 0.3 torr which corresponds to 94% Hgo removal in bimolecular and 6% removal in termolecular processes, according to the rate coefficients given above. The emission profile of the stabilised monomer was then scaled appropriately and subtracted; the residual spectrum supposedly due solely to reaction (2) is indicated by curve b of fig. 1.
15 January 1977
CHEMICAL PHYSICS LETTERS
2. Experimental
Wavelength Clt)
Fig. 1. Spectral profiles: (a) stab&cd monomer; (b) the spectrum with NH3 at 0.3 ton; (c) the 3000 A band of Hikida et al. IS].
More recently Hikida et al. [S] published spectra with NH, pressures down to 0.08 torr of NH,. They showed that the spectral profile changes quite significantly as the pressure is reduced below 0.3 torr, which is in conflict with the analysis of Callear and Connor. Hikida et al. conclude that at low pressures the emission is comprised of spectra a and c of fig. 1 and that the observed profiles are a linear superposition, but not in proportion to prediction from “well established” rate coefficients k2 and k,. The interesting conflict may be seen to have two aspects. First, according to published rate coefficients the 0.3 torr spectrum corresponds to 94% bimolecular removal of Hgo by NH, and therefore it should change very little if the pressure is decreased further. Secondly, Hikida et al. observed equal emission intensities of the two bands a and c at about 0.08 torr, at which pressure 98% of the Hgo removal occurs in the bimolecular reaction (2). They suggest that the spectrum c arises from a complex formed between Hg(6 3Pr) and NH,. Emission via reactions (2) and (3) was supposed to have the common profile a. The intensity of the emission is very weak below 0.1 torr of NH,. Hikida et al. devised a special method to obtain high spectral purity for cw excitation at 2537 a; spectra were recorded with photon counting.
We describe here a detailed investigation of the low pressure emission with a time resohed technique with which we have demonstrated that all the fluorescence observed by Callear and Connor, and by Hikida et al_ arises fro& interaction of HgO with NH3. Negligible luminescence occurs from reactions of Hg(6 3P,)_ The light source was a transverse discharge lamp, length 20 cm diameter 3 cm, with 5 electrode pairs. The Iamp was repetitively pulsed, usually at 10 Hz and operated at ~0.1 J per flash. The quartz reaction vessel was not located along the lamp axis as in previous designs [3 J , but was situated 15 cm from the lamp and coupled to it with an aluminium reflector. At low pressures, the electric fields can initiate discharge in the reaction volume with the axial design. The reaction vessel was constructed with a filter jacket, and formed part of a closed loop around which the gases were continuously circulated over mercury. The lamp was filled with He at 1 torr plus H, at . 0.1 torr. Emission was measured with an EM1 6256 B photomultiplier, racked on a Hilger and Watts medium quartz spectrograph. Signals were averaged, typically over lo3 flashes, with a Data Lab transient recorder (DL. 920) and signal averager (DL.403). Ni experiments were conducted-at 293 (23) K.
3. Results and discussion Photographs of some oscilloscope traces are shown in fig. 2. Fig. 2a is the 2537 A time profile from the reaction vessel containing only Hg vapour, and is equivalent to the flash profile. Emission at 2940 A is shown in fig. 2b with the reaction vessel empty (background) and in fig. 2d after addition of NH3 at 1.30 torr. The peak of the background
emission
corresponds
to the
delay time at which Hg(6 3Pr) is at its maximum,
and is unaffected by addition of NH3. However, in fig. 2d a delayed emission is evident. The profile of fig. 2c was obtained with the same gas filling as fig. 2d but with the monitoring wavelength shifted to 3500 a. The relaxation times of the delayed fluorescence in figs. 2c and 2d are indistinguishable. At high ammonia pressures, when the Hgo relaxation time is below 0.1 PS, the 2940 A emission foflows the flash profile. However, at high pressure the 3500 A fluorescence is al-
205
.
Fig. 2. Tie resolved emission profiles, all 200 IIS Cull scale: (a) the 2537 A flash duration;(b) background at 2940 A; (c) delayed emission at 3500 A, with NH3 at 1.3 torr; (d) delayed emission at 2940 A, with NH3 at 1.3 torr.
ways of longer duration than the flash profile, which
we attribute to the finite radiative lifetime of a stabilised complex. Thus, with a time resolved teclmique, we have been able to establish that the emission at both 2940 a and 3500 A is due to reactions of Hgo, the latter being associated at least partly with stabilised monomer. We next examined the totd emission at the two wavelengths varying the NH, pressure between Ct.03 torr and 30 torr. It was discovered that the ratio of the total emission at 3500 J%to that at 2940 ,!k is proportional to [NH,], which is ifiustrated in fig. 3. This simple feature indicates that the short wavelength fluorescence occurs by the same mechanism over a
wide pressure range, and obviously allows the possibility that the 2940 A emission is due to reaction (2), and that at 3500 a is due to reaction (3). ‘fhere are, however, other schemes which can account for the observed features. The short wavelength emission could, for example, be due to Hg%H~ formed in a termolecular coliision, which has not suffered vibrational releation in subsequent collisions. To examine the mechanisms in further depth, we have recorded the full emission profiles down to below Q-1 torr. The spectra agree well with those published by Hikida et al., and we agree with the separation into two bands as given in their fig. 2. We have also measured the intensity maxima at 3500 A and 2940 II as a function of [NH3], finding the former to be proportional to [NH3f3 anir the latter to @II&J2 below 0.5 torr. These results are presented in fig. 4 as plots of log intensity versus log [NHs]. The lines drawn througl the data points have slopes of 3 at 3500 A and 2 at 2940 A. The Gg. 4 data support the simple scheme in which the 3500 a bared is assigned to the stabilised monomer and the 2940 A emission to unstabilised complex, formed in a bimolecular collision. At low hressure the fraction of Hg(6 3P,) quenched becomes proportions to [NH,], sothat the product [Hgo ] [NH3 J, which controls the bimolecular intensity, is proportional to [NH,]2_ Similarly the rzte of formation of s&biked complex is proportional to [Hgo] [NH21 2, which becomes proportional to [NH31 3 at low pressure. The Xhreshold of the bimolecular emission is 2650 a, corresponding to the full excitation energy of HgO. 20
I+()
z
2
c:
;5
00 2
I
-1s
I -10
I
1
I
-05 0 log,,(tNH,l/Torr~
*OS
?lO
l
-1.0 -10
-0 75
-0.50
-0.25
Fig. 3. The ratio of the intensityat 2940 A to that at 3500
A, multiplied by the pressure of NH3 (torr). The high pressure measurements (solid points) were made at 28ClLlA and have been scaled to correspond to 2940 A accordingto spec
trum c offii. 1. 206
Fig. 4. Piots of log intensity versus log pressure below OS
torrr (a) data points 2940 A emission;(b) data points 3500
A emission.
it remained to discover why the ratio of bimolecuIar tcr term&c&r emission does not correspond to that predicted from t&e p~~~~s~~drate coefficients_ To this end, and also in reIat.ion to other aspects of this research, we have critically examined the previous methods of deriving k, and some sets of measurement: have been repeated. With the phase shift and flash techniques, the reciprocal of the Fig0 relaxation time, r-l, was assumed to be given by r-1 -kz[NH:,]
+@Et#
*
Rate coefficients were detived from intercepts and slopes of a plot ctf(~$NEQf)-~ versus [NH3f - At Iow pressures, huwever, we now believe that a sigrdficant fraction of the HgOis lost-by diffusion to the walls, the effect of which Brasenhanced the magnitudes of the intercepts, and hence the derived I&J*by about 3 fold. Full details will be reported elsewhere. Accurding to the revised rate coefficients at 0.08 torr of NH,, 90% af the HgOshould be removed in bimolecular events, and 10% in terrnolecuiar. This revision of the bimolecular rate coefficient does not eliminate the curnflict out&red above. To account for the observation that the emissiun intensities are approximately equal at 0.08 torr, it is necessary to postulate that the quantum yield of bimolecufar emission is small (MM), reiative ts the termolecufar emission process. The main route for bimolecular removal of Hgn by NH3 must be radiationless, probably
The b&nokxxdar emission may OCCXK by way of an inverted ~red~ss~iatiun~ crossing from the A2 state which ccnrefates with Hgu, to a state correlating with Hg(6 3P,) which emits the 3000 a band c of fig_ I_
Acknowledgement C.G.F. is grateful to the Royal Society for the award of a Royal Society and Nuffield Foundation Cummonwealth Burssry. We thank the Science Research Council for an e+ipment grant-
References C.G.Fr~eman,M.J.McEwan, R,F.C.CitidgeandL.F. Phillips, Chem. Fhys. Letters 9 (197 1) 578. CC. Freeman, M.J. McEwan, R.F,C. Gltidge and L.F. Phillips, Trans. Farad+y Sot. 67 (1971) 2004. A.B. Callear and J.C. McGurk, Chem.. Phys. Letters 7 (1970) 49 1. A.B. Calkar, J-H_Connor and J. Koskikdllio, J. Chem. Sot. Faraday 1170 <1974) 1542. A-B- Harker and CS. Burton, f. Chem. Fltys. 63 f19751 885. K_ Luther, H.R Wendt and R-E_ Hunziker, them. Fhys. Letters 33 11975) 146. A.% C&Hear and J.C. Connor, J. C&em. Sot. Faraday If 70 (1974) 1667. T, Mikida, T. Ichimura and Y. MO& Chem. Phys. Letters 27 (1974) 548.
Hgt-,+NHs-+NH2+HgH.
207
Volume 45, number 2
THE EXCITED MERCURY-AMMONIA Anthony
15 January 1977
EMISSION AT LOW PRESSURE
and Colin G. FREEMAN*
B. CALLEAR
Physical Chemistry Department. Carnbndgc CB2 ZiP. UK
University
of Cambridge,
Received 4 October I976
The luminescence from Hg, NH3 mixtures, when excited with 2537 A resonance radiation, has been investigated at low pressures (> 0.03 torr) with a time resolved technique. It is concluded that the carrier of the band with a maximum at = 3000 A is an I&NH: complex formed in a bimolecular encounter of Hg(6 3Po) with NH3. The quantum yield of the bimolecular emission is small (O.l), the main channel for bimolecular removal of Hg(6 3Po) being radiaticnless.
1_ Introduction
HgNH; + Hg + NH, + hv _
When mixtures are ejrcited-with
let luminescence an intensity
OF ammonia
acd mercury
vapour
2537 a resonance radiation, ultraviooccurs via a broad diffuse band, with
maximum
at about
3500
A.
For the most part, it appears that the emissicn is associated with the interaction of Hg(6 3Po) (hereafter Hgo) with NH, _With a phase shift technique [ 1,2], it has been established that the emission is delayed relative to the excitation at 2537 A; Hg(6 3P,), formed initially, undergoes spin orbit relaxation to Hgo which is removed slowly at low pressure to generate the delayed fltiorescence Hg(63P1)+NH3+Hg0+NH3,
k,
Hgo &NH3 :
Hg+NH3
(1) +hv.
(2)
Relaxation times of Hgo and decay constants of the ultraviolet emission were found to be indistinguishable with a flash photolysis method [3], fcr 7 2 20 ps; from an analysis of the effect of inert gases it was established that removal of HgO by NH3 also occurs in termolecuIar co&ions: Hgo +NH,
+M
-+HgNH;
+M,
(3)
* On leave from the University of Canterbury, Christchurch, New Zealand.
204
(4)
With high pressures of NH,, the luminescence lags behind the Hgo decay profile because of the finite radiative lifetime of the stabilised HgNH; complex of = 1.8 J.S. Rate coefficients derived from phase shifts [ 1,2], flash photolysis experiments [4], and phase sensitive modulation spectroscopy ES,6 1, are in good agreement. The bimolecular coefficient k2 has been reported to be 3.2 (kO.3) X IO-l3 cm3 molecule-’ s-l and the termolecular coefficient k, (M = NH$2.2 (20.3) X 10m30 cm6 molecuIe-3 s-l at about 293 K. The spectral profile of the luminescence varies with pressure of NH, and also with pressure of added inert gas. In this letter we are not concerned with effects zt high pressures of ammonia where Hgo may attach clusters of NH, [7] _ The emission profile with NH3 at about 1 torr, and with an atmosphere of added N is 2,’ almost entire!y due to stabilised monomer, HgNH,. This spectrum is illustrated by curve a of fig. 1. To examine the emission from HgNH; formed via reaction (2), Callear and Connor [7], using cw excitation, recorded the spectrum with NH3 at 0.3 torr which corresponds to 94% Hgo removal in bimolecular and 6% removal in termolecular processes, according to the rate coefficients given above. The emission profile of the stabilised monomer was then scaled appropriately and subtracted; the residual spectrum supposedly due solely to reaction (2) is indicated by curve b of fig. 1.
15 January 1977
CHEMICAL PHYSICS LETTERS
2. Experimental
Wavelength Clt)
Fig. 1. Spectral profiles: (a) stab&cd monomer; (b) the spectrum with NH3 at 0.3 ton; (c) the 3000 A band of Hikida et al. IS].
More recently Hikida et al. [S] published spectra with NH, pressures down to 0.08 torr of NH,. They showed that the spectral profile changes quite significantly as the pressure is reduced below 0.3 torr, which is in conflict with the analysis of Callear and Connor. Hikida et al. conclude that at low pressures the emission is comprised of spectra a and c of fig. 1 and that the observed profiles are a linear superposition, but not in proportion to prediction from “well established” rate coefficients k2 and k,. The interesting conflict may be seen to have two aspects. First, according to published rate coefficients the 0.3 torr spectrum corresponds to 94% bimolecular removal of Hgo by NH, and therefore it should change very little if the pressure is decreased further. Secondly, Hikida et al. observed equal emission intensities of the two bands a and c at about 0.08 torr, at which pressure 98% of the Hgo removal occurs in the bimolecular reaction (2). They suggest that the spectrum c arises from a complex formed between Hg(6 3Pr) and NH,. Emission via reactions (2) and (3) was supposed to have the common profile a. The intensity of the emission is very weak below 0.1 torr of NH,. Hikida et al. devised a special method to obtain high spectral purity for cw excitation at 2537 a; spectra were recorded with photon counting.
We describe here a detailed investigation of the low pressure emission with a time resohed technique with which we have demonstrated that all the fluorescence observed by Callear and Connor, and by Hikida et al_ arises fro& interaction of HgO with NH3. Negligible luminescence occurs from reactions of Hg(6 3P,)_ The light source was a transverse discharge lamp, length 20 cm diameter 3 cm, with 5 electrode pairs. The Iamp was repetitively pulsed, usually at 10 Hz and operated at ~0.1 J per flash. The quartz reaction vessel was not located along the lamp axis as in previous designs [3 J , but was situated 15 cm from the lamp and coupled to it with an aluminium reflector. At low pressures, the electric fields can initiate discharge in the reaction volume with the axial design. The reaction vessel was constructed with a filter jacket, and formed part of a closed loop around which the gases were continuously circulated over mercury. The lamp was filled with He at 1 torr plus H, at . 0.1 torr. Emission was measured with an EM1 6256 B photomultiplier, racked on a Hilger and Watts medium quartz spectrograph. Signals were averaged, typically over lo3 flashes, with a Data Lab transient recorder (DL. 920) and signal averager (DL.403). Ni experiments were conducted-at 293 (23) K.
3. Results and discussion Photographs of some oscilloscope traces are shown in fig. 2. Fig. 2a is the 2537 A time profile from the reaction vessel containing only Hg vapour, and is equivalent to the flash profile. Emission at 2940 A is shown in fig. 2b with the reaction vessel empty (background) and in fig. 2d after addition of NH3 at 1.30 torr. The peak of the background
emission
corresponds
to the
delay time at which Hg(6 3Pr) is at its maximum,
and is unaffected by addition of NH3. However, in fig. 2d a delayed emission is evident. The profile of fig. 2c was obtained with the same gas filling as fig. 2d but with the monitoring wavelength shifted to 3500 a. The relaxation times of the delayed fluorescence in figs. 2c and 2d are indistinguishable. At high ammonia pressures, when the Hgo relaxation time is below 0.1 PS, the 2940 A emission foflows the flash profile. However, at high pressure the 3500 A fluorescence is al-
205
.
Fig. 2. Tie resolved emission profiles, all 200 IIS Cull scale: (a) the 2537 A flash duration;(b) background at 2940 A; (c) delayed emission at 3500 A, with NH3 at 1.3 torr; (d) delayed emission at 2940 A, with NH3 at 1.3 torr.
ways of longer duration than the flash profile, which
we attribute to the finite radiative lifetime of a stabilised complex. Thus, with a time resolved teclmique, we have been able to establish that the emission at both 2940 a and 3500 A is due to reactions of Hgo, the latter being associated at least partly with stabilised monomer. We next examined the totd emission at the two wavelengths varying the NH, pressure between Ct.03 torr and 30 torr. It was discovered that the ratio of the total emission at 3500 J%to that at 2940 ,!k is proportional to [NH,], which is ifiustrated in fig. 3. This simple feature indicates that the short wavelength fluorescence occurs by the same mechanism over a
wide pressure range, and obviously allows the possibility that the 2940 A emission is due to reaction (2), and that at 3500 a is due to reaction (3). ‘fhere are, however, other schemes which can account for the observed features. The short wavelength emission could, for example, be due to Hg%H~ formed in a termolecular coliision, which has not suffered vibrational releation in subsequent collisions. To examine the mechanisms in further depth, we have recorded the full emission profiles down to below Q-1 torr. The spectra agree well with those published by Hikida et al., and we agree with the separation into two bands as given in their fig. 2. We have also measured the intensity maxima at 3500 A and 2940 II as a function of [NH3], finding the former to be proportional to [NH3f3 anir the latter to @II&J2 below 0.5 torr. These results are presented in fig. 4 as plots of log intensity versus log [NHs]. The lines drawn througl the data points have slopes of 3 at 3500 A and 2 at 2940 A. The Gg. 4 data support the simple scheme in which the 3500 a bared is assigned to the stabilised monomer and the 2940 A emission to unstabilised complex, formed in a bimolecular collision. At low hressure the fraction of Hg(6 3P,) quenched becomes proportions to [NH,], sothat the product [Hgo ] [NH3 J, which controls the bimolecular intensity, is proportional to [NH,]2_ Similarly the rzte of formation of s&biked complex is proportional to [Hgo] [NH21 2, which becomes proportional to [NH31 3 at low pressure. The Xhreshold of the bimolecular emission is 2650 a, corresponding to the full excitation energy of HgO. 20
I+()
z
2
c:
;5
00 2
I
-1s
I -10
I
1
I
-05 0 log,,(tNH,l/Torr~
*OS
?lO
l
-1.0 -10
-0 75
-0.50
-0.25
Fig. 3. The ratio of the intensityat 2940 A to that at 3500
A, multiplied by the pressure of NH3 (torr). The high pressure measurements (solid points) were made at 28ClLlA and have been scaled to correspond to 2940 A accordingto spec
trum c offii. 1. 206
Fig. 4. Piots of log intensity versus log pressure below OS
torrr (a) data points 2940 A emission;(b) data points 3500
A emission.
it remained to discover why the ratio of bimolecuIar tcr term&c&r emission does not correspond to that predicted from t&e p~~~~s~~drate coefficients_ To this end, and also in reIat.ion to other aspects of this research, we have critically examined the previous methods of deriving k, and some sets of measurement: have been repeated. With the phase shift and flash techniques, the reciprocal of the Fig0 relaxation time, r-l, was assumed to be given by r-1 -kz[NH:,]
+@Et#
*
Rate coefficients were detived from intercepts and slopes of a plot ctf(~$NEQf)-~ versus [NH3f - At Iow pressures, huwever, we now believe that a sigrdficant fraction of the HgOis lost-by diffusion to the walls, the effect of which Brasenhanced the magnitudes of the intercepts, and hence the derived I&J*by about 3 fold. Full details will be reported elsewhere. Accurding to the revised rate coefficients at 0.08 torr of NH,, 90% af the HgOshould be removed in bimolecular events, and 10% in terrnolecuiar. This revision of the bimolecular rate coefficient does not eliminate the curnflict out&red above. To account for the observation that the emissiun intensities are approximately equal at 0.08 torr, it is necessary to postulate that the quantum yield of bimolecufar emission is small (MM), reiative ts the termolecufar emission process. The main route for bimolecular removal of Hgn by NH3 must be radiationless, probably
The b&nokxxdar emission may OCCXK by way of an inverted ~red~ss~iatiun~ crossing from the A2 state which ccnrefates with Hgu, to a state correlating with Hg(6 3P,) which emits the 3000 a band c of fig_ I_
Acknowledgement C.G.F. is grateful to the Royal Society for the award of a Royal Society and Nuffield Foundation Cummonwealth Burssry. We thank the Science Research Council for an e+ipment grant-
References C.G.Fr~eman,M.J.McEwan, R,F.C.CitidgeandL.F. Phillips, Chem. Fhys. Letters 9 (197 1) 578. CC. Freeman, M.J. McEwan, R.F,C. Gltidge and L.F. Phillips, Trans. Farad+y Sot. 67 (1971) 2004. A.B. Callear and J.C. McGurk, Chem.. Phys. Letters 7 (1970) 49 1. A.B. Calkar, J-H_Connor and J. Koskikdllio, J. Chem. Sot. Faraday 1170 <1974) 1542. A-B- Harker and CS. Burton, f. Chem. Fltys. 63 f19751 885. K_ Luther, H.R Wendt and R-E_ Hunziker, them. Fhys. Letters 33 11975) 146. A.% C&Hear and J.C. Connor, J. C&em. Sot. Faraday If 70 (1974) 1667. T, Mikida, T. Ichimura and Y. MO& Chem. Phys. Letters 27 (1974) 548.
Hgt-,+NHs-+NH2+HgH.
207