Lifetime studies of the HgNH3 complex

Volume 55, number 2

CHEMICAL OHYSICS LETTERS

15 April 1978

LIFETIME STUDIES OF THE HgNH, COMPLEX T. HIKIDA, M. SANTOKU and Y. MORI Depamnent of Ckemïmy. Tokyo Instituteof Technology. Meguro-ku, Tokyo, Jqnn Received 4 November 1977 Revïsed manuscript received 20 December 1977

The hnninescence hrduced by the Hg-photosensitized reaction of NHa was studïed by repetitive fast puked excitation. From observations of the decay of the luminescente, the emitter of the 290 nm fluorescente was found to be the precursor of the 340 nm emksion (the stabied complex). The fnst-order decay rate of the stabihzed complex was found to he given by I/re +k[NHa], where re = 2.3 1~sand k = 3.5 X 10-13cm3 molecule-’ sA1 -

1. Introduction

HgNH3 + Hg(l So) + NH3 + 340 run emission .

The spectrum of the mercury photosensitïzed lunúnescence of ammonia consists of two ïntensity maxima at around 305 and around 340 nm [ 1-4]_ These emission bands have been assigned to the electronïc transitions starting from different excited states of the HgNHs complex; one witb higher energy whïch correlates with P&(3Pl) and another wïth lower energy which correlates with Hg(3Po). Recently, Callear and Freeman [3,4] have investigated the time-resolved emission using flash excitation at 253.7 nm, the mercury resonance line. They have concluded that the sbort wavelengtb emission band may be assigced to the unstabilized complex which results from a bimolecular reaction of Hg(3Po) with NH3 by way of an inverted predissociation to the bigher excited state, and that the long wavelength component is attributed to the stabibzed complex formed by a termolecular reaction of Hg(3Po) and NH3 :

Also, they have shown that the ratio of the time íntegral of the emission ïntensity obstuved at 350 nm to that at 294 nm increased lïnearly with the pressure of NH3 _ The short wavelength emission predomïnated at the limiting zero pressure. Wüs seems to be consistent with the bi- and termolecular mechanism. An investigation of the sensitized luminescente of the Hg-NH3 (and ND3) system with stationary excitation at 253.7 nm by the present authors [ 1,2] again seems to confirm tbis mechanism. Recently, we have developed a fast 253.7 nm flash lamp of bigb repetition rate which permits us to reinvestigate the time-resolved lurnïuescence of tbe HgNH3 complex with nanosecond time resolution. In tbis paper we describe measurements of the fluorescente intensity as a function of the time and the pressure of NH3, in order to obtain a deeper understanding of the mercury photosensitized luminescente of NH3, especially in the role of Hg(3P1) on the short wavelength component.

Hg(3P1) + NH3 -f Hg(3P0> + NH3 , HgcPu)

+ NH3 -+ Hg(l S,,) + NH3

+ 305 nm emission,

Hg(3Po) f 2NH3 -f HgNH3 + NH, ,

280

(1)

2. (2) (3

(4)

Experimental

The apparatus c0nsïst.s of a lïght pulser for excitation, a reaction cell, and a detectïon system. A fiee runníng high voltage discharge type ligbt pulser is constructed in a coaxïal eometry including a

Volume 55, number 2

15 April 1978

CHJZMICAL PHYSICS LETTERS

Fig. 1. Design of the light puker: A. resistor (8 Mn); B, chargïng capacitor; C. discharge lamp contaìnïng Hg and Ar gas; D, trigger gap-

resistor, a coaxial capacitor forming a transmission line, a discharge lamp, and a trigger gap, as shown in fig. 1. The discharge lamp is made of fused silica contabüng 50 torr of Ar gas and a droplet of Hg. It is usually opcrated at 8-10 kV with a repetition rate of 1-10 kHz depending on the supplied voltage and the gap distance. Maïn features of the spectral output of the light pulser are the 184.9 and 253.7 run resonance lines of Hg and some weaker Hg lines are also observed. These are practically the same as those of a low pressure mercury lamp. The full width at half maximum of the lïght pulse at 253.7 run is about 40 ns with a decay constant of about 30 ns. The half widths of the light puLses at fS4.9,3 13, and 366 mn are similar and have values of about 10 ns. The reaction cell is made of fused silica with its volume about 300 ml. The fluorescente was observed at right angles to the pulsed excitation light beam with a combination of a monochromator and a photomultiplier. The fluorcscence decay was measurcd wïth the technique of single photon counting 15-71 using a time-to-amplitude converter and a multichannel analyser. NH3 was handled as described previously [ 11. AU experiments were carried out at 297 -C1 K.

2

0

L

nme (psec)

6

Fig. 2. Fluorescente decay curve of HgNH3 complex. (a) The points are the measured fluorescente decay curve at 290 run with [NHa] = 14.5 torr. The line is the curve convoluted from curve (b) and qe “best-fit” decay rate constant of 5.0 X 10’ s-l. (b) “Excitïng light pulse” observed at 253.7 nm in the presence of NHs, 14.5 torr.

[3,4,8]. Between 290 and 320 mn, the time profiles were complex and depended on the wavelength. This may be understood as the superposition of the two different time profiles, those of the unstabiied and the stabilized complex [8]. A time profiíe observed at 290 run with NH, at 14.5 torr is shown in fig. 2, curve a(points). Curve b is the tbme profde of the light intens@ observed at 253.7 nm scattered from Hg(3Pr) atoms in the cell containing NH3 at 14.5 torr. If the fluorescente at 290 nm (unstabilized complex) results from the reaction of Hg(3P1 ) with NH,, the thne profile of the fluorescence intensity at 290 nm, 12go(f), ís @ven by the time convolution 19, IO] of the time profile of the Hg(3P1) concentration, c(t), wïth a fwst-order decay rate constant, k;

3. Results and discussion The time profìles of the fluorescente intensity observed beween 270 and 290 nm were quite similar with a faster time response compared wïth those at longer wavelengths. At longer wavelengths, between 320 and 360 mn, the time profìles were the same although the time response was slower than these at shorter wavelengths. These results correspond to an emission spectrum consisting of two bands, the faster tune response to the emission band at shorter wavelength (the unstabílized complex) and slower one to that at longer wavelength (the stabílízed complex)

1290 (r) = j

c(r - r’) e-kr’ dt’ .

(5)

0

Curve a(lïne) is a time convolution of curve b with the “best-fit” decay rate constant of 5.0 X LOS.s-l_ Although the result of the convolution (curve a, line) well reproduces the observed time profile, this does not necessarily indicate that the fluorescente of the unstabïlïzed complex results directly from Hg(3P1) and NH3. Hg(3Po) should aIso be formed by collïsions between Hg(3 PI) and NH3, and be quenched by NH3. The decay rate constant in the presence of NH3 at 281

Volume 55, number 2

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Oo

1

20

1

30

40

1 50 tNH-$

, 60

torr Fig. 3. Plot of “best-fit” decay rate constant (290 nm) versus square of NH3 pressure.

14.5 torr is estimated to be 6.0 X 105 s-l using reported quencbing rates [ 11-151 of Hg(3Po) by NH3. Thus, the tirne evolution of the concentration of Hg(3Po) atoms in the cell sbould be close to curve a in fig. 2. Computed “best-fit” first-order decay rate constants of tbe luminescente intensity at 290 run at various pressures of NH3 are found to be a linear function of

[NH312, which is shown in fig. 3. The slope of this linear function is 1.4 X 10m30 cm6 molecule-2 s-l_ This may be compared to tbe reported quencbîng rate constants of Hg(3Po) by NH3 [ 11-151, although our

vake is slightly smaller. The ïntercept is probably due to the dîffusional 10s.~ of Hg(3Po) atoms. The biiolecular quenching of Hg(3Po) by NH, is not significant within the pressure range studied [2,4]. These experimental results lead to the conclusion tbat the time profdes of the short wavelength emission band approxïmately represent the time varïation of the Hg(3Po) concentration- It is diffïcult to believe that the Iifetïme of the unstabïlized complex is of the order of m, and also that its quenching reaction is in the third-order. The short wavelength emission band has been assigned to the hïgher electronic state which correlates with Hg(3P1) [2-4]. Its lifetime is probably very short [ 1,151, if the complex has enough energy to decompose to Hg(3Po) and NH3. A statistical equilibrium should be established between the complex and HgcPo) + NH3 by way of vibronïc interaction with the iower state wbïch correlates wïth Hg(3Po) [4] _ A detailed measurement of the earlier portion of the 290 nm emission with a fäster time resolution in282

15 April 1978

CHEM ICAL PHYSICS LETTJZRS

Fíg. 4. Fluorescente decay cwve of HgNH3 complex at 340 om, [NH3] = 14.5 torr. The points are the experimental flu-

orescence decay curve. The line is the curve convoluted from curve (a) of fig. 2 and the “best-fit” decay rate constant of 6.0 x 10’

s-l .

dicated no significant deviation from the time proíìle obtained by the convolution of the 253.7 nm fluorescence with a proper time constant for the pressure of NH3 between 5 torr and 68 torr even in the period of the maximum intensity of the 253.7 nm fiuorescence. Thïs indïcates that the lifetime of the complex between Hg(3P1) and NH3 is very short and confirms that the contribution of the complex between Hg(3P1) and NH3 to the 290 MI emission, at the NH, pressures investigated, is negligible as has been indîcated by Callear and Freeman [4]. Time profiles of the emission from the lower electronk state observed at 340 nm with NH3 at 14.5 torr are shown in fig. 4 (points). The “best-fit” decay rate of 6.0 X los s-l is computed by convolution of the 290 run emission (fig. 2, curve a). The qualïty of the fit is excellent. This indicates that the errütter of the 290 nm fluorescente is the precursor of the 340 nm ernission and also that the time profile (fïg- 2, curve a) approximately

represents

tbe time evolution

of tbe

Hg(3Po) concentration. First-order decay rate constants of the 340 nm enGsion were studied as a fünction of NH3 pressure. Fig. 5 is a plot of the “best&” first-order decay rate constant versus the pressure of NI&. This plot extrapolated to zero pressure yields a lifetime of 2.3 &s. The radiative Wetime of the stabilized complex has been measured wïth phase shift IS, 11,16-181 and with flash éxcitation [15,18]. AU data are slightly smaller than our vahre. The origïn of the conflict remains unclear but it may be due to the dif-

CHEUICAL

Volume 55. number 2

PHYSICS LETTERS

15 April 1978

References

111T. Hitida, T. Ichimura and Y. Mori, Chem. Phys. Letters 27 (; 974) 548.

121T. Hikîda, T. Ihihara and Y. Mori, Chem. phys. Letters

I

20

I

I

60

Hg. 5. Plot of “best-fit” decay rate constant (340 nm) versus pressureof NH3.

ference in the pressure of NH,. The slope of the plots of the decay rate versus pressure gìves the quenching rate constant of the stabilized complex by NH3 which is 3.5 X 10-13 cm3 molecule-l s-~. On the contrary, Caliear et al. [15] have reported that the lifetime of the 340 nm emission was hardly affected by the NH3 pressure between 20 and 750 torr. They have analysed the tïme profde of the 350 run emïssion by shnple convolution of an approximated exponential pulse shape of = 2 @. The present analysis, however, bas been performed by the convolution of the time profile of the 290 nm emissïon, the concentration of the precursor of the 340 nm emíssion. This seems to be a dominant source of the conflict. Moreover, the emission spectrum at the high NHs pressure was attributed to attachment of fùrther NH3 groups to the HgNH, complex, though similar lifetimes are expected for these complexes [ lg]_

52 (1977) 43. 131 A.B. Callear and C.G. Freeman, Chem. F’hys.Letters 45 (1977) 204. 141 A.B. Callear and CG. Freeman, Chem. Phys. 23 (1977) 343. L51 B. Bicksand LH. Mumo, in: Progressin reaction kiietics, Vol. 4. ed. G. Porter Qergamon, Oxford, 1967). 161 W.R. Ware, in: Creation and detection of the excited state, Vol. lA, ed. A.A. Lamola (Dekker. New York, 1971). [71 A.E.W_ Knight and B.K. Selinger, Australian J. Chem. 26 (1973) 1. IS1 H. Umemoto. S. Tsunashimaand S. Sato, Chem. Phys. Letters 53 (1978) 521. 191 L. Hundley, T. Cobum, E. Ganvh and L. Stryer, Rev. Sci. Instr. 38 (1967) 488. IW J.N. Demas and G.A. Crosby, Anal. Chem. 42 (1970) LOLO1111 C.G. Freeman, M.J. McEwan, R.F.C. Clarïdge and L.F. Phillïps, Chem. Phys. Letters 9 (1971) 578. 1121 CG. Freeman, MJ. McEwan, R.F.C. Claridge and L.F. Phiips, Trans. Faraday Sec. 67 (1971) 2004. El31 A.B. Callear and J.C_McGurk, Chem. Phys. Letters 7 (1970) 491. 1141 A.B. Callear and J.C. McGurk, 1. Chem. Sec. Faraday 11 69 (1973) 96. r151 A.B. Callear, J.H. Connor md J. Koskïkallïo, J. Chem. Sec. Faraday 1170 (1974) 1542. 1161 A.B. Harker and C.S. Burton, J. Chem. Phys. 63 (1975) 885. 1171 K. Luther, H.R. Wendt and H.E. Hunziker, Chem. Phys. Letters 33 (1975) 146. 1181 J. Koskikallio, A.B. Callear and J.H. Connor, Chem. Phys. Letters 8 (1971) 467. Wl A.B. Callear and J.H. Connor, J. Chem. Sec. Faraday 11 70 (1974) 1667.

Acknowiedgement We thank Professor S. Sato for givìng us much useful information and results prior to publication.

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