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Absolute yields of I(2P12 ) in I2 photodissociation using a laser optoacoustic technique
145
Chemical Physics 111 (1987) 145-153 North-Holland, Amsterdam
ABSOLUTE YIELDS OF I( * P,,,) IN I, PHOTODISSOCIATION USING A LASER OPTOACOUSTIC TECHNIQUE Tom F. HUNTER
and Christine
M. LEONG
School of Chemical Sciences, Universip of Eust Angliu, Norwich
Received
13 September
1985; in final form 22 September
NRI
3RB. UK
1986
Using the argon-ion laser lines in the region 457.9-514.5 nm for excitation, followed by optoacoustic measurements and analysis, the absolute yields of I( 2P,,2) in the photodissociation of I, have been measured wavelength. A comparison with other techniques is made. The yields of I( 2P,,z) for excitation below dissociation limit are discussed.
1. Introduction In wavelength regions of absorption where several transitions contribute to the overall absorption cross section one useful method of examining the nature of these transitions is to examine the states of any products formed. This is particularly used for the halogens and the alkyl halides where the photodissociation, in certain wavelength regions, produces ground state (*Ps,*) and excited state (*Pi,*) halogen atoms in proportions which allow correlation to be established with the original absorbing state or states. The measurement of ( *Pi,* )/( *P3,* ) branching ratios as a function of wavelength, in addition to this spectroscopic information it gives, has the practical value of determining the wavelength region over which lasing can occur for the appropriate halogen atom. For such atomic lasers the I atom has been of most significance, with sufficient gain shown for a number of alkyl iodides [l] and with I, itself [2] in the narrow excitation region 493-501 nm. Determination of the (2P1,2)/( * Pj,*) branching ratio (or, of q, the absolute quantum yield of I( 2Pi,2) following photodissociation) has normally involved direct quantitative measurements on the produced I( *Pj,*) or I( *Pi,*) using atomic absorption or atomic emission in the case of the latter state. For the alkyl iodides, results on q 0301-0104/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
time-dependent as a function of the 12(B state)
have been very scattered and the discovery [3,4] of complications due to exciplex, R * I( *Pi,*), formation (R is the parent alkyl iodide) and emission does not seem [5] to have settled the problem. Results for q as a function of wavelength in CH,I, CD,& CF,I [6,7], CH,I, [8] and n-C,F,I [9] have been obtained by an optoacoustic method which is absolute at each wavelength and not dependent on absorption coefficient data. In general, these measured optoacoustic q values have been somewhat lower than those recorded using atomic spectroscopy, and, in particular, for the molecule n-C,F,I which has commonly [lO,ll] been assumed to have a q of unity across its 7~* + n transition, the optoacoustic q was measured [9] with a maximum of 0.72 and with strong variation with absorption wavelength. One of the main reasons for the present work is to carry out similar optoacoustic measurements on I, photodissociation where a number of previous experiments [12-161 have established a reasonable consensus for photodissociation parameters in the region 430-510 nm. Would the optoacoustic values be low relative to these I, results? In the visible region of interest, three transitions from the ground state ‘Xl, X, are found, i.e. to the repulsive state, ‘II,,, the bound state, 311,,u, B, and the slightly bound state, 3111u, A. The B state correlates with I(2Pi,2) plus I(*P,,,), with a dissociation energy equivalent to 20043 cm-’ [17], B.V.
whilst the other states correlate with two ground state atoms. In this region Tellinghuisen [14] has resolved the absorption spectrum into the three transitions with peaks at 498.5 (‘III,, + X), 530 (B +- X) and 673 nm (A +- X), but disagreement is still possible with I, as shown by the magnetic circular dichroism results [18] with the same three peaks found at 508, 538 and 725 nm respectively. The present optoacoustic results pertain to six argon-ion laser lines from 457.9-514.5 nm and absorption in this .region is exclusively to the B and the ‘IIlu states [14]. Previous results on photodissociation in I, in this region, and in particular the formation of I( 2P1,2) will be examined in detail in section 4 (for example, see fig. 6 for previous q values).
2. Optoacoustic model The energy available for translational and electronic excitation of the I atoms following photodissociation is (h Y - D) where hv is the energy of the absorbed photon and D is the dissociation energy of the X state of I,. Any translational energy produced directly is distributed very quickly [19] throughout the gaseous system (in these experiments, always a low pressure of I, plus 50 Torr He). Such translational energy has to equilibrate with any internal, available modes in the system, i.e. the rotations and vibrations of I,. Under the experimental conditions where 50 Torr He is present and where the optoacoustic modulation frequency is always < 3 kHz, rotationaltranslational energy transfer is much too fast [20] to contribute any phase lags to the optoacoustic measurements. The vibrational levels are also available, the first state lying only 213 cm-’ above the ground state. For 1,-I, collisions the V-T rate has been measured by Hunter and Kristjansson [21] as 1.68 X lo-’ s atm at 295 K (the present measurements are done at room temperature), and, using pulsed molecular beams, Hall et al. [22] have measured a V-T relaxation time of = lo-’ s atm for I,-He collisions. Under the present experimental conditions this means that any transfer of energy from the excess translational energy produced in the photodissociation to the vibrational
modes of I, is too fast to cause any observable phase lags (i.e. < 0.5 “); this result is substantiated in measurements by Kristjansson [23] who observed no optoacoustic phase lags in I,--Kr mixtures (0.3 Torr/60 Torr) up to modulation frequencies of 3 kHz. Helium should be a more efficient V,T relaxer than Kr. The above discussion leads to the conclusion that heat release in the system will be due to two factors: (a) the very fast initial translational energy, and (b) the relaxation of any I* (i.e. I( 2P1,2)) produced in the dissociation. The decay of I*, having an Einstein coefficient for spontaneous emission of only 9 s-l [24], is dominated by bimolecular radiationless processes which produce heat to be measured optoacoustically. The three possible collision partners in these measurements are He, I, and I with measured rate constants for I* deactivation of < 5 x lo-‘s [25], = 3 x lo-” [28] cm3 moleculee’ [26,27] and = 1.6 X lo-l4 s-l respectively. In these modulated excitation experiments the concentration of I is sufficiently low to ensure that I* decay is dominated by I, collisions. For the two heat-yielding processes, (a) and (b), then, the concentration of I, can be so arranged as to ensure that process (b) lies in the experimental time (frequency) range and process (a) is very much faster than this. This is a very common situation for optoacoustic relaxation measurements and has been detailed in the literature [6,19]. The optoacoustic relaxation phase lag is given by tan 6 =
w7 1 + (H/L)(l
+ W’?)
’
(1)
where o is the angular frequency of the modulation of the incident beam of radiation, r is the lifetime of I*, and H/L is the ratio of the heat energy released in process (a) to that in (b), i.e. the ratio of fast to slow energy release. Optoacoustic cells, designed for accurate phaselag measurements [29], are constructed from metal, and, although this gives no problems for most gases, for the halogens and in particular I, the affinity for the metal walls is such that no steady I 2 pressure is available over any satisfactory period of time. Thus the normal measurement of 8 at a wide variety of experimental w values, followed
T. F. Hunter, C. 154.Leong / Ahsolutevields
by calculation of 7 and H/L using eq. (l), is not possible. However, the pressure of I, in the cell, once closed, falls at a steady rate and a variant of eq. (1) involving the pressure p of I,, is available, viz. tan 9 =
l%/P 1 + (H/L)(l + w’$/p’)
’
(2)
where the pressure involved is p, the pressure of I,, since it is I, which is responsible for the I * deactivation. ~a is the lifetime of I* at unit pressure of I,. Provided some means is available (see later) for the measurement of relative I, pressures at each measurement of 0 then H/L can be measured and from H/L the desired value of q can be calculated. For calculation of a secondorder rate constant for deactivation of I * by I,, absolute, and not relative, values of I, pressure are necessary. The basis of this optoacoustic method, then, lies in eq. (2) with measurement of 0 at various, relatively known, I, pressures, p. Two further points, however, should be made in this section. 2. I. Relationship of H/L
to q
Provided that the irradiation of the I,-He system produces a dissociative yield of unity (see section 4 for analysis of this point for the two excitation wavelengths 501.7 and 514.5 nm both of which lie below the B state dissociation limit), then q is the fraction of all dissociations producing I + I*. With 1 - q that fraction producing 21, then the fast release of energy, H, is given by
H=(l-q)(hv-D)+q(hv-D-E), or
H=hv-D-qE, where E is the I*, I energy gap equivalent cm- ‘. Similarly,
L = qE
(3) to 7603
(4)
giving
hv - D ‘= E(l+H/L)’
14-l
of I( ‘P,,_,) in I, photodissociution
The energies E and D are very accurately known, as are the photodissociative energies of the particular Ar+ laser lines, hv. Thus establishment of H/L defines q for any wavelength of excitation without any reference to other parameters such as absorption coefficients, etc. 2.2. Small correction for increase in particle density
in photodissociation Following the absorption of radiation the main contribution to the acoustic pressure comes from the flow to translational energy as discussed above. However, in any dissociation there is an increase in particle density in the system and, for modulated incident radiation, this gives rise to oscillatory pressure changes and thus additional acoustic amplitude. The acoustic signal at the microphone, Pi(w), for particle density changes, has been previously analysed in considerable detail [30] and is given by Pi(w)
= [RTFcos(+&‘G,J] Xexp[i(
wt - IT/~ - #I~)],
(6)
where R is the gas constant, T is the absolute temperature, F is the amplitude of the oscillatory part of the absorbed radiation, N is Avogadro’s number and &, is defined by tan & = ~(kdl), with k, the rate constant for the dissociation. Both for direct dissociation and any predissociation in these experiments, k, is very much faster than the time scale set by the frequency range used, and thus (pd -+ 0 and cos +d + 1. This particle density acoustic signal, Pi( w ), has to be added to the main translational energy based signal, P2(w ), which is given by
P,(u)=(RFA/cco) where
exp[i(wt--a/2-t9)],
C is the heat capacity
(7)
of the gas per mole,
A is given by A = ( H2 + L2m2 + 2HLm2)“2,
(5)
with m = (1 + w272)-1/2, are as previously defined,
and the other see eq. (1).
symbols
T. F. Huuter, C.M. teong / Ahsolute~~ields of I( ‘P,/)
148
The analysis [30] on particle density effects indicated that Pi(w) is normally much less than P2(w). However, to achieve the full expression these two are vectorially added with the result that the observed final phase lag, e,, is given by tan f3, =
A sin I3 AcosB+K’
where K = TC/N. Expressing this in terms appropriate to the desired variable pressure analysis (see eq. (2)) gives W%/P
tan t9, = 1 + [(H+
K)/L](l
which is seen to be very similar correction K included. The signal amplitude R is R = ( A2 + K* + 2AK
+ t&;/p”)
’
(9)
to eq. (2) with the
COS d)l’*.
The main method of measurement used is to measure the maximum phase lag as the pressure of I, falls in the cell, and, from eq. (9) (tan e,,,),,=
{2[(H+
K)/L]“*
x[I+(H+K)/L]“~}-~.
(10)
For 50 Torr of added He dominating the heat capacity C and with a temperature of measurement of 293 K, K = 306 cm-‘. H + L is equivalent to hv - D, varying with the chosen wavelength of excitation, and a determination of (e,,,),, yields H/L and thus q (from eq. (5)).
I)ZI, photodissociotion
The He was BOC Research Grade, 99.9995% pure, and the I, was purified by multiple “ freeze-pumpthaw” cycles plus sublimations. Typical I, pressures were of the order of 50 mTorr initially, falling to around 5 mTorr. The optical arrangement was as shown in fig. 1. The Spectra Physics 170 series Ar + laser was used at several watt output for the most powerful lines, and the lines used for I, photodissociation were 457.9, 476.5, 488.0, 496.5, 501.7 and 514.5 nm. Calibration of such acoustic measurements is obviously critical and requires the measurement of phase in a gas showing as closely as possible the properties of the gas under study but with no relaxation phase lag present. An ideal solution is present in these measurements since change to longer wavelengths, using a dye laser, enables the exact I,/He mixture under study to be used for calibration, i.e. the same gas mixture is used but with no I* production or relaxation and thus calibration should be exact within the limits of accuracy of the measurement. No relaxation time delays at the frequency of modulation used must be present in the calibration. A strong peak in the banded region of the I, spectrum at 599 nm was chosen (an I, spectrum in the R6G dye region, 565-645 nm, was run and wavelength calibrated using the opto-galvanic effect with a Na/Ne hollow cathode lamp). From previous work [12,22] no time-delays, or phase lags, are present on the experimental time scale either from the lifetime of the electronically excited states or from any subsequent vibrational excitation due to the translationally hot I atoms produced (see earlier discussion on vibrational-translational relaxation times in the presence of 50 Torr He). For a particular sample and excitation wave-
3. Experimental The optoacoustic cell used was similar to that described for other measurements [6,29], being machined from stainless steel with a volume of 10 cm3 and a pathlength of 3 cm. A special needlevalve plus bellows arrangement ensured a completely smooth inner surface when the cell was closed; no problems due to Helmholtz resonance were thus present. Extensive anti-vibration measures were taken.
VACWM SV%TEM
My Ar+ LASER
b EA
Fig. 1. splitter, position ulator,
Simplified experimental arrangement. BS is beam M is high reflectivity mirror, MM is movable doublemirror, PM is photomultiplier, IM is intensity modand EA is electronic analysers of microphone signal.
T. F. Hunter,
C. M. Leong / Absolute .vields of I(‘P,j2)
length, consecutive measurements were carried out using Ar+ and dye laser excitation by suitable positioning of the movable mirror MM in fig. 1. In fig. 1, all mirrors are high reflectance (> 99%) and the beam splitter, BS, gave 80-90% transmittance between 570 and 620 nm and = 98% reflectance at 514-450 nm, both for 45O incidence. Any small drift in phase measurement over an extended period of time was corrected for by determining the phase of the modulation of the laser beam using a photo-multiplier as shown in fig. 1.
149
in I> photodissociation
30
__I-i-i--1--_,
- I.
-1.1
_-I.,” %
V
o
F
.I.
-‘=”
207y+ , 10
,
0
,o’
0
/O’
0
SC
o--o-o
-
/O’
‘0,
‘0.
c
-0.
-0
,
100
200 Time / mm
Fig. 3. Examples of plots of the relaxation phase lag, 0,. as a function of time for excitation by Ar+ laser lines at (a) 476.5 nm, (b) 501.7 nm, and (c) 514.5 nm.
4. Results and discussion With phase lag measured in a clockwise direction, from the experimental procedure given in section 3, the desired relaxation phase lag 6, is given by
where 19,~ and eMP refer to phases measured on the photomultiplier and microphone respectively. As shown in eq. (10) the quantity of interest in the measurement of q is (e,,,),,, and figs. 2 and 3 show examples of typical plots of 0, against time of run. This is equivalent to t9, versus pressure of I, but the time axis is not linear in pressure; this is
of no consequence since it is only the value of e,,, at its maximum which is of interest. An example of the way in which the pressure of I, falls in the cell once the cell is closed is shown in fig. 4, for some typical dosing; the I, relative pressure is taken from the amplitude of the dye laser acoustic signal. Fig. 2 shows values of 0, for 488 nm excitation, indicating the range of curves for different dosing characteristics. By “different dosing characteristics” is meant the following. If a pressure of I, is left in the cell overnight prior to the
I , 6-
01 0
I
1
1
100
200
300
Time/ min
Fig. 2. Plots of the relaxation phase lag, t$,,, as a function of time after the cell is closed, for excitation by Ar? laser line at 488.0 mn. The different plots represent different dosing characteristics by I, prior to the measurement. The maximum 0, is the same, within experimental error, in each case.
Time /hours
Fig. 4. Typical fall of dye laser induced acoustic signal amplitude (arbitrary units) as a function of time. The signal amplitude is directly proportional to I, pressure in the cell.
150
T. F. Huplter, C.M. Leo~tg /
Absoluteyields of I(‘P,,,)
experiment then the cell walls are well covered and the rate of loss of I, to the walls in the actual experiment is slow. If the cell has been strongly evacuated prior to the experiment then I, adsorption will be much faster. In table 1 are given (e,,,),, values for the various wavelengths, the value at 457.9 nm being less accurate because the various I, dosing characteristics tried did not give so clear a maximum as for the other excitation wavelengths. Another method of measurement is exemplified by results taken at 496.5 nm excitation and shown in fig. 5. From eq. (9) it can be seen that a plot of ( p tan 8,))’ versus p-’ should be linear and that from the subsequent intercept and gradient (H + K)/L is available. The relative I, pressure is sufficient in this analysis and plot (fig. 5) and such pressures are measured from the dye laser induced acoustic amplitude; p is therefore in arbitrary units. The resultant q values in table 1 come from the listed H/L data using eqs. (10) and (5) and the (hv - 0) values given in the table. K has a constant value of 306 cm-’ in each case. The q values are also plotted in fig. 6 for comparison with previous work, and it is obvious that the present optoacoustic measurements give results in essential agreement with other work involving determinations of I and I* concentrations. The minimum value of q for I atom lasing to occur is 0.67 (due to the degeneracy of the 2P1,2 and 2P3,2 states and the production of two I atoms from I*) and, within experimental error, this value is achieved at both 496.5 and 501.7 nm. Lasing is actually observed [2] from 493 to 501 nm, and it is
Table 1 Results for the various parameters, see text x (nm)
(k),, (de&
457.9 416.5 488.0 496.5 501.7 514.5
7.5 12.5 23.0 nr 29.5 9.5
” (K + H)/L
is found
parameters
used in the calculation
(K + H)/L
1)~I, photo~issocirrtio,l
0
‘/P2 Fig. 5. Plot of (p tan 6,) ’ versusp -*. where p is I, pressure in arbitrary units and 0, is relaxation phase lag, for excitation with Ar+ laser line at 496.5 nm.
obvious that the required population of I* is only just obtained; the I atom laser from photolysis of I, is operating very much on threshold. It is useful at this stage to indicate briefly how q values were obtained from previous work and thus presented for comparison in fig. 6. Oldman et al. [12] used photofragment spectroscopy and distinguished clearly the extent of absorption from the X state to the B and lIIlu states (the A state does not absorb in the wavelength range covered by them). The measured ratios of ‘IIIIU + X to B + X, with the q value in parentheses, were 1.2 + 0.2 (0.46 k 0.04), 1.3 k 0.1 (0.44 -t 0.02) and 2.3 f 5 (0.31 f 0.05) at 479.6, 464.9 and 449.8 nm respectively. Comparison with the present values at 457.9 and 476.5 nm gives excellent agreement. Broadbent and Callaer [13] found the quantum yield of I* at four excitation wavelengths using the amount of alkyl iodide formation in mixtures
of 4, the quantum
H/L
yield
of I( ‘P t,*)
hu(cm
i 2.0 i_ 1.0 i_ 1.0 * 1.0 * 1.0
3.81k 1.20 1.83 *0.19 0.78 + 0.05 0.65 + 0.08 0.52 + 0.03 2.56 kO.32
from data in fig. 5.
40
20
3.66+ 1.02 1.73iO.18 0.72 i 0.05 0.5910.08 0.46 i 0.03 2.41 I 0.31
D
-I )
9399 8546 8052 7701 7492 6996
formation.
For definitions
Y 0.29 0.41 0.62 0.64 0.68 0.27
f f f + + f
0.06 0.03 0.02 0.03 0.02 0.03
of
T. F. Hunter, C. M. Lemg / A hsolute vi&Is
10
-I
+ 0,
I,
440
I
I
460
I,
I
I
I
I
I
480
I
500
I
,.
I.
0 1
520
h /nm
Fig. 6. Quantum field of I( 2P,,2) production, q, as a function of excitation wavelength, X. ( I) is present results, and the other references arc as follows: (X) [12], (0) [13] in He, (0) [32], (+) [15] and (A) [16]. The horizontal line represents the threshold for lasing.
of I,, propane and inert gas. The q values they obtained at the wavelengths 483.5, 502.0, 515.5 and 533.0 nm were respectively 0.49, 0.41, 0.28 and 0.08 for He as the inert gas, and 0.47, 0.47, 0.31 and 0.03 with Xe. The values at 483.5 and 502.0 nm are considerably lower than the present values, although the 515.5 nm value is within experimental error of that measured at 514.5 nm in this work. Tellinghuisen [14] produced careful I, absorption data in this wavelength region and from his values for ‘IIt” + X vis-a-vi, B +- X absorption strengths, values of q can be calculated with the assumption that the ‘IIlu state only produces 21 and the B state only I* plus I. Such values are shown in fig. 6 over the range 440 nm (q = 0.39) to 510 nm (q < 0.74). Both Burde et al. [15] and Wiessenfeld and Young [16] used pulsed tunable dye laser excitation and followed the extent of I or I* spectroscopically. These q values should be the most accurate of the previous data and should also be useful in the comparison of techniques mentioned in section 1, i.e. in measurements on alkyl iodides the optoacoustic results seem lower than those obtained by spectroscopic measurements on I and I * concentrations. The q values obtained are given in fig. 6, but, for more accurate comparison, the highest values measured by Burde et al. [15] were 0.63 and 0.63 at 495 and 500 nm. Wiesenfeld and Young [16] also got their highest values in this
of I( ‘P,,_,)
iu I2 photodissociution
151
region, namely 0.667 and 0.682 at 495 and 498 nm respectively. In both cases [15,16] the agreement is good with the optoacoustic data at 496.5 and 501.7 nm, well within the experimental error, and there seems no reason to suspect that the optoacoustic technique has an inherent tendency to produce low results. The conclusion is that on studies of photofragmentation in alkyl iodides the optoacoustic results are likely to be fairly accurate and that the high values obtained by I atom spectroscopic techniques (and the value of unity for q often assumed for n-C,F,I photolysis) are inaccurate for some reason; see section 1. Such lower values for q for alkyl and fluoro-alkyl iodides have significant modelling implications for the development of iodide lasers pumped by radiation from the sun and of possible application in space. I(‘P,,,) formation at energies below the B state dissociation limit. The dissociation limit of the B state is at a wavenumber of 20043 m- ‘. Two of the wavelengths used in this study, 501.7 nm (19932 cm-‘) and 514.5 nm (19436 cm-‘) are thus at energies where extra energy is required to explain the q values reported. Other workers [13,15,16,31,32] have made similar observations. It has been suggested [33] that production of I(2Pr,2) below the dissociation limit may be, at least in part, due to B + X absorption from “hot bands”, i.e. some of the absorption is from thermally excited rotational and rovibrational levels of the X state. This is certainly possible for the results of Callear et al. [13,31,32], who carried out the main study below the dissociation limit, where continuous radiation with interference filters was used. However, in the present case the details of absorption from the two laser lines at 501.7 and 514.5 nm have been studied [34-361. With 501.7 nm excitation a number of B + X rovibrational transitions are possible [35], all originating in u” = 0, and more than 90% of the structured absorption (i.e. to the B state) is due to 62-0, R26 [34,35]. The extra energy, E, required to reach the dissociation limit with 501.7 nm excitation is 111 cm-‘. Using, for I*(X), values [37] of the rotational constant of 3.737 X lo-* cm-’ and for the centrifugal distortion constant of 4.25 x lop9 cm-‘, the dominant transition only provides an
152
T. F. Hunter, C. M. Lemg / Absoluie~vields of I( ‘P,/_,) 1~ I_, photodssociutim
extra 26 cm-‘, and other sources, presumably collisional effects on I,(B state), must be involved. For 514.5 nm excitation e is 607 cm- ’ and [34,36] more than 90% of the absorption is in 43-0, P13 and R15. These two respectively only contribute 7 and, although the small extent of and 9 cm-’ other transitions may be more important, any contribution from thermally populated levels is obviously very much less than 607 cm-‘. It therefore appears certain that, in the present measurements at 501.7 and 514.5 nm, no contribution to the measured q comes from direct dissociation. In the region of these two wavelengths, and with the collisional effect of 50 Torr He present, previous work [38-401 shows that fluorescence is negligible; the only two processes of importance are I,(B) + 21 in spontaneous and collisional predissociation and I,(B) -+ I + I * in some collisional release mechanism. The initial levels excited are B, u’ = 62 with 501.7 nm excitation and B, u’ = 43 with 514.5 nm excitation, and the He collisions may spread this energy to close-lying vibrational levels before the above important dissociative channels operate [40]. The measured extent of I* formation is 0.68 and 0.27 at 501.7 and 514.5 nm respectively. The smoothed data of Tellinghuisen [33] give the fractional absorption to the B state at 0.67 and 0.77 at these two wavelengths, which yields for the quantum efficiency, say F, from the B state alone the values of 1.01 and 0.35 at 501.7 and 514.5 nm. The relative contributions of ‘IIlu + X and B + X given by Tellinghuisen [33] may not be very accurate in the banded region for specific laser lines, but simple extrapolation from the continuous region suggests a substantial presence of the ‘Hi,, + X absorption. Other authors, viz. Burde et al. [15] using high-resolution dye laser excitation, have accepted the Tellinghuisen data in this wavelength region. Because of the nature of the present excitation the high F values are obviously due to excited state collisions. Although produced, or selected, in a cool rotational state, the 50 Torr He ensures that rotational thermalisation of the excited I, molecules will occur. In the work of Collear et al. [13] high pressures of added gas were also present; their result at 515.5 nm is very close to that
measured here at 514.5 nm although at a similar wavelength to 501.7 nm their result is lower by around a factor of 0.7 (see fig. 7). The suggestion [41] that complex formation between the excited I, molecule and the additive contributed available energy to the I, bond rupture (thus making I* formation possible) was discounted when Broadbent and Collear [13] found similar efficiencies for a variety of additives, viz. He, Xe, C,H,, H,, N,, Ar and CO,. The remaining possibility that the physical degrees of freedom in the I,(B) + M interaction contribute efficiently in producing I* is thus a very likely explanation. Considering the relative translation, the I, internal rotation and the orbited rotation of I, relative to M [13], the probability P that energy greater than E is available in the system is given by P = [t(~/kT)~
+ c/kT+
l] exp( -r/kT).
(11)
P represents a maximum since all such energy could not be expected to contribute to release of I*; nevertheless in fig. 7, where the solid plot is of P, the results at 501.7 and 514.5 nm are seen to give good agreement. Burde et al. [15], using low pressures of I, alone, found values below both the P plot and the present values (fig. 7). Since I*(X)
0,4@2-
W&V, 500
I 510
I 520
. I
I: I 530
h/m
Fig. 7. F values for excitation of I, at energies below the I l(B state) dissociation limit. F is quantum yield of I( 2P1,2) formation following B state excitation. ($) is present result, (0) [15], (x) [13] in He, (+) [13] in Xe. The solid line is calculated using eq. (11).
T. F. Hunter. C. M. Leong / Absolute .vields of I(2P,,2)
collisions are known to have essentially unit efficiency [42] in producing either 21 or I + I*, in experiments with I, alone present, no rotational thermalisation of I,(B) will take place. It would appear therefore that the dye laser excitation of Burde et al. [ 151 must produce I,(B) in a cooled rotational state, or, if rotational energy is not the important factor, the propensity for producing I* must be less in It(X) than in He collisions, i.e. the branching ratio ( *PI,* )/( *P3,* ) in I*(B) + M + I (’ P3,2 ) + (I(2p3,2) orI(2P1,2))
+ M
is higher for the inert gas. Neither the calculated P factor of eq. (11) nor arguments based on the polarizability of M fit in with this. It is obvious that the extent of I(*P,,,) formation at wavelengths below the B state dissociation limit requires a rather more careful experimental and theoretical investigation.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15]
M.A. Pollack, Appl. ,Phys. Letters 8 (1966) 36. S.J. Davis, Appl. Phys. Letters 32 (1978) 656. E. Gerck, Opt. Commun. 41 (1982) 102. E. Gerck, J. Chem. Phys. 79 (1983) 311. J.E. Smedley and S.R. Leone, J. Chem. Phys. 79 (1983) 2687. T.F. Hunter and KS. Kristjansson, Chem. Phys. Letters 58 (1978) 291. T.F. Hunter, S. Lunt and K.S. Kristjansson, .I. Chem. Sot. Faraday II 79 (1983) 303. T.F. Hunter and K.S. Kristjansson, Chem. Phys. Letters 90 (1982) 35. T.F. Hunter and C.M. Leong, Chem. Phys. Letters 104 (1984) 538. T. Donohue and J.R. Wiesenfeld, J. Chem. Phys. 63 (1975) 3130. J.B. Koffend and S.R. Leone, Chem. Phys. Letters 81 (1981) 136. R.J. Oldman, R.K. Sander and K.R. Wilson, J. Chem. Phys. 54 (1971) 4127. T.W. Broadbent and A.B. Callear, J. Chem. Sot. Faraday II 68 (1972) 1367. J. Tellinghuisen, J. Chem. Phys. 59 (1973) 849. D.H. Burde, R.A. MacFarlane and J.R. Weisenfeld, Phys. Rev. A 10 (1974) 1917.
ill I2 photodissociution
153
and R.H. Young, Chem. Phys. 58 (1981) 51. and P. Luc, J. 1171 J.W. Tromp, R.J. LeRoy, S. Gerstenkom Mol. Spectry. 100 (1983) 82. I181 M. Brith, M.D. Rowe, 0. Schnepp and P.J. Stephens, Chem. Phys. 9 (1975) 57. 1191 T.F. Hunter and P.C. Turtle, in: Advances in infrared and Raman spectroscopy, Vol. 7, eds. R.J.H. Clark and R.E. Hester (Heyden. London, 1980) ch. 5. and rotational relaxation in WI J.D. Lambert, Vibrational gases (Oxford Univ. Press. Oxford, 1978). WI T.F. Hunter and K.S. Kristjansson, J. Chem. Sot. Faraday II 75 (1979) 1670. WI G. Hall, K. Liu, M.J. McAuliffe, C.F. Giese and W.R. Gentry, J. Chem. Phys. 78 (1983) 5260. ~31 K.S. Kristjansson, Ph.D. Thesis, University of East Anglia (1980). I241 F.J. Comes and S. Pionteck, Chem. Phys. Letters 42 (1976) 558. 1251 R.J. Donovan and D. Husain, Trans. Faraday Sot. 62 (1966) 1050. WI I. Arnold, F.J. Comes and S. Pionteck, Chem. Phys. 9 (1975) 237. 1271 H. Hofmann and S.R. Leone, J. Chem. Phys. 69 (1978) 641. WI R.J. Donovan and D. Husain, Chem. Rev. 70 (1970) 489. 1291 T.F. Hunter, D. Rumbles and M.G. Stock, J. Chem. Sot. Faraday 70 (1974) 1010. 1301T.F. Hunter and K.S. Kristjansson, J. Chem. Sot. Faraday II 75 (1975) 1284. [311 T.W. Broadbent, A.B. Callear and H.K. Lee, Trans. Faraday Sot. 64 (1968) 2320. ~321T.W. Broadbent and A.B. Callear, Trans. Faraday Sot. 67 (1971) 3030. I331 J. Tellinghuisen, J. Chem. Phys. 59 (1973) 849. I341 J.B. Koffend, R. Bacis and R.W. Field, J. Mol. Spectry. 77 (1979) 202. 1351 J.P. Pique, F. Hartmann, R. Bacis and S. Churassy, Opt. Commun. 36 (1981) 354. [361 J.A. Paisner and R. Wallenstein, J. Chem. Phys. 51 (1974) 4317. [371 R.F. Barrow and K.K. Yee, J. Chem. Sot. Faraday II 69 (1973) 684. [381 R.L. Brown and W. Klemperer, J. Chem. Phys. 41 (1964) 3072. I391 J.I. Steinfeld and W. Klemperer, J. Chem. Phys. 42 (1965) 3475. WI R.B. Kurzel and J.I. Steinfeld, J. Chem. Phys. 53 (1970) 3293. 1411J.J. Deakin and D. Husain, J. Chem. Sot. Faraday II 68 (1972) 1603. 1421 J.I. Steinfeld and A.N. Schweid, J. Chem. Phys. 53 (1970) 3304.
1161J.R. Wiesenfeld
Chemical Physics 111 (1987) 145-153 North-Holland, Amsterdam
ABSOLUTE YIELDS OF I( * P,,,) IN I, PHOTODISSOCIATION USING A LASER OPTOACOUSTIC TECHNIQUE Tom F. HUNTER
and Christine
M. LEONG
School of Chemical Sciences, Universip of Eust Angliu, Norwich
Received
13 September
1985; in final form 22 September
NRI
3RB. UK
1986
Using the argon-ion laser lines in the region 457.9-514.5 nm for excitation, followed by optoacoustic measurements and analysis, the absolute yields of I( 2P,,2) in the photodissociation of I, have been measured wavelength. A comparison with other techniques is made. The yields of I( 2P,,z) for excitation below dissociation limit are discussed.
1. Introduction In wavelength regions of absorption where several transitions contribute to the overall absorption cross section one useful method of examining the nature of these transitions is to examine the states of any products formed. This is particularly used for the halogens and the alkyl halides where the photodissociation, in certain wavelength regions, produces ground state (*Ps,*) and excited state (*Pi,*) halogen atoms in proportions which allow correlation to be established with the original absorbing state or states. The measurement of ( *Pi,* )/( *P3,* ) branching ratios as a function of wavelength, in addition to this spectroscopic information it gives, has the practical value of determining the wavelength region over which lasing can occur for the appropriate halogen atom. For such atomic lasers the I atom has been of most significance, with sufficient gain shown for a number of alkyl iodides [l] and with I, itself [2] in the narrow excitation region 493-501 nm. Determination of the (2P1,2)/( * Pj,*) branching ratio (or, of q, the absolute quantum yield of I( 2Pi,2) following photodissociation) has normally involved direct quantitative measurements on the produced I( *Pj,*) or I( *Pi,*) using atomic absorption or atomic emission in the case of the latter state. For the alkyl iodides, results on q 0301-0104/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
time-dependent as a function of the 12(B state)
have been very scattered and the discovery [3,4] of complications due to exciplex, R * I( *Pi,*), formation (R is the parent alkyl iodide) and emission does not seem [5] to have settled the problem. Results for q as a function of wavelength in CH,I, CD,& CF,I [6,7], CH,I, [8] and n-C,F,I [9] have been obtained by an optoacoustic method which is absolute at each wavelength and not dependent on absorption coefficient data. In general, these measured optoacoustic q values have been somewhat lower than those recorded using atomic spectroscopy, and, in particular, for the molecule n-C,F,I which has commonly [lO,ll] been assumed to have a q of unity across its 7~* + n transition, the optoacoustic q was measured [9] with a maximum of 0.72 and with strong variation with absorption wavelength. One of the main reasons for the present work is to carry out similar optoacoustic measurements on I, photodissociation where a number of previous experiments [12-161 have established a reasonable consensus for photodissociation parameters in the region 430-510 nm. Would the optoacoustic values be low relative to these I, results? In the visible region of interest, three transitions from the ground state ‘Xl, X, are found, i.e. to the repulsive state, ‘II,,, the bound state, 311,,u, B, and the slightly bound state, 3111u, A. The B state correlates with I(2Pi,2) plus I(*P,,,), with a dissociation energy equivalent to 20043 cm-’ [17], B.V.
whilst the other states correlate with two ground state atoms. In this region Tellinghuisen [14] has resolved the absorption spectrum into the three transitions with peaks at 498.5 (‘III,, + X), 530 (B +- X) and 673 nm (A +- X), but disagreement is still possible with I, as shown by the magnetic circular dichroism results [18] with the same three peaks found at 508, 538 and 725 nm respectively. The present optoacoustic results pertain to six argon-ion laser lines from 457.9-514.5 nm and absorption in this .region is exclusively to the B and the ‘IIlu states [14]. Previous results on photodissociation in I, in this region, and in particular the formation of I( 2P1,2) will be examined in detail in section 4 (for example, see fig. 6 for previous q values).
2. Optoacoustic model The energy available for translational and electronic excitation of the I atoms following photodissociation is (h Y - D) where hv is the energy of the absorbed photon and D is the dissociation energy of the X state of I,. Any translational energy produced directly is distributed very quickly [19] throughout the gaseous system (in these experiments, always a low pressure of I, plus 50 Torr He). Such translational energy has to equilibrate with any internal, available modes in the system, i.e. the rotations and vibrations of I,. Under the experimental conditions where 50 Torr He is present and where the optoacoustic modulation frequency is always < 3 kHz, rotationaltranslational energy transfer is much too fast [20] to contribute any phase lags to the optoacoustic measurements. The vibrational levels are also available, the first state lying only 213 cm-’ above the ground state. For 1,-I, collisions the V-T rate has been measured by Hunter and Kristjansson [21] as 1.68 X lo-’ s atm at 295 K (the present measurements are done at room temperature), and, using pulsed molecular beams, Hall et al. [22] have measured a V-T relaxation time of = lo-’ s atm for I,-He collisions. Under the present experimental conditions this means that any transfer of energy from the excess translational energy produced in the photodissociation to the vibrational
modes of I, is too fast to cause any observable phase lags (i.e. < 0.5 “); this result is substantiated in measurements by Kristjansson [23] who observed no optoacoustic phase lags in I,--Kr mixtures (0.3 Torr/60 Torr) up to modulation frequencies of 3 kHz. Helium should be a more efficient V,T relaxer than Kr. The above discussion leads to the conclusion that heat release in the system will be due to two factors: (a) the very fast initial translational energy, and (b) the relaxation of any I* (i.e. I( 2P1,2)) produced in the dissociation. The decay of I*, having an Einstein coefficient for spontaneous emission of only 9 s-l [24], is dominated by bimolecular radiationless processes which produce heat to be measured optoacoustically. The three possible collision partners in these measurements are He, I, and I with measured rate constants for I* deactivation of < 5 x lo-‘s [25], = 3 x lo-” [28] cm3 moleculee’ [26,27] and = 1.6 X lo-l4 s-l respectively. In these modulated excitation experiments the concentration of I is sufficiently low to ensure that I* decay is dominated by I, collisions. For the two heat-yielding processes, (a) and (b), then, the concentration of I, can be so arranged as to ensure that process (b) lies in the experimental time (frequency) range and process (a) is very much faster than this. This is a very common situation for optoacoustic relaxation measurements and has been detailed in the literature [6,19]. The optoacoustic relaxation phase lag is given by tan 6 =
w7 1 + (H/L)(l
+ W’?)
’
(1)
where o is the angular frequency of the modulation of the incident beam of radiation, r is the lifetime of I*, and H/L is the ratio of the heat energy released in process (a) to that in (b), i.e. the ratio of fast to slow energy release. Optoacoustic cells, designed for accurate phaselag measurements [29], are constructed from metal, and, although this gives no problems for most gases, for the halogens and in particular I, the affinity for the metal walls is such that no steady I 2 pressure is available over any satisfactory period of time. Thus the normal measurement of 8 at a wide variety of experimental w values, followed
T. F. Hunter, C. 154.Leong / Ahsolutevields
by calculation of 7 and H/L using eq. (l), is not possible. However, the pressure of I, in the cell, once closed, falls at a steady rate and a variant of eq. (1) involving the pressure p of I,, is available, viz. tan 9 =
l%/P 1 + (H/L)(l + w’$/p’)
’
(2)
where the pressure involved is p, the pressure of I,, since it is I, which is responsible for the I * deactivation. ~a is the lifetime of I* at unit pressure of I,. Provided some means is available (see later) for the measurement of relative I, pressures at each measurement of 0 then H/L can be measured and from H/L the desired value of q can be calculated. For calculation of a secondorder rate constant for deactivation of I * by I,, absolute, and not relative, values of I, pressure are necessary. The basis of this optoacoustic method, then, lies in eq. (2) with measurement of 0 at various, relatively known, I, pressures, p. Two further points, however, should be made in this section. 2. I. Relationship of H/L
to q
Provided that the irradiation of the I,-He system produces a dissociative yield of unity (see section 4 for analysis of this point for the two excitation wavelengths 501.7 and 514.5 nm both of which lie below the B state dissociation limit), then q is the fraction of all dissociations producing I + I*. With 1 - q that fraction producing 21, then the fast release of energy, H, is given by
H=(l-q)(hv-D)+q(hv-D-E), or
H=hv-D-qE, where E is the I*, I energy gap equivalent cm- ‘. Similarly,
L = qE
(3) to 7603
(4)
giving
hv - D ‘= E(l+H/L)’
14-l
of I( ‘P,,_,) in I, photodissociution
The energies E and D are very accurately known, as are the photodissociative energies of the particular Ar+ laser lines, hv. Thus establishment of H/L defines q for any wavelength of excitation without any reference to other parameters such as absorption coefficients, etc. 2.2. Small correction for increase in particle density
in photodissociation Following the absorption of radiation the main contribution to the acoustic pressure comes from the flow to translational energy as discussed above. However, in any dissociation there is an increase in particle density in the system and, for modulated incident radiation, this gives rise to oscillatory pressure changes and thus additional acoustic amplitude. The acoustic signal at the microphone, Pi(w), for particle density changes, has been previously analysed in considerable detail [30] and is given by Pi(w)
= [RTFcos(+&‘G,J] Xexp[i(
wt - IT/~ - #I~)],
(6)
where R is the gas constant, T is the absolute temperature, F is the amplitude of the oscillatory part of the absorbed radiation, N is Avogadro’s number and &, is defined by tan & = ~(kdl), with k, the rate constant for the dissociation. Both for direct dissociation and any predissociation in these experiments, k, is very much faster than the time scale set by the frequency range used, and thus (pd -+ 0 and cos +d + 1. This particle density acoustic signal, Pi( w ), has to be added to the main translational energy based signal, P2(w ), which is given by
P,(u)=(RFA/cco) where
exp[i(wt--a/2-t9)],
C is the heat capacity
(7)
of the gas per mole,
A is given by A = ( H2 + L2m2 + 2HLm2)“2,
(5)
with m = (1 + w272)-1/2, are as previously defined,
and the other see eq. (1).
symbols
T. F. Huuter, C.M. teong / Ahsolute~~ields of I( ‘P,/)
148
The analysis [30] on particle density effects indicated that Pi(w) is normally much less than P2(w). However, to achieve the full expression these two are vectorially added with the result that the observed final phase lag, e,, is given by tan f3, =
A sin I3 AcosB+K’
where K = TC/N. Expressing this in terms appropriate to the desired variable pressure analysis (see eq. (2)) gives W%/P
tan t9, = 1 + [(H+
K)/L](l
which is seen to be very similar correction K included. The signal amplitude R is R = ( A2 + K* + 2AK
+ t&;/p”)
’
(9)
to eq. (2) with the
COS d)l’*.
The main method of measurement used is to measure the maximum phase lag as the pressure of I, falls in the cell, and, from eq. (9) (tan e,,,),,=
{2[(H+
K)/L]“*
x[I+(H+K)/L]“~}-~.
(10)
For 50 Torr of added He dominating the heat capacity C and with a temperature of measurement of 293 K, K = 306 cm-‘. H + L is equivalent to hv - D, varying with the chosen wavelength of excitation, and a determination of (e,,,),, yields H/L and thus q (from eq. (5)).
I)ZI, photodissociotion
The He was BOC Research Grade, 99.9995% pure, and the I, was purified by multiple “ freeze-pumpthaw” cycles plus sublimations. Typical I, pressures were of the order of 50 mTorr initially, falling to around 5 mTorr. The optical arrangement was as shown in fig. 1. The Spectra Physics 170 series Ar + laser was used at several watt output for the most powerful lines, and the lines used for I, photodissociation were 457.9, 476.5, 488.0, 496.5, 501.7 and 514.5 nm. Calibration of such acoustic measurements is obviously critical and requires the measurement of phase in a gas showing as closely as possible the properties of the gas under study but with no relaxation phase lag present. An ideal solution is present in these measurements since change to longer wavelengths, using a dye laser, enables the exact I,/He mixture under study to be used for calibration, i.e. the same gas mixture is used but with no I* production or relaxation and thus calibration should be exact within the limits of accuracy of the measurement. No relaxation time delays at the frequency of modulation used must be present in the calibration. A strong peak in the banded region of the I, spectrum at 599 nm was chosen (an I, spectrum in the R6G dye region, 565-645 nm, was run and wavelength calibrated using the opto-galvanic effect with a Na/Ne hollow cathode lamp). From previous work [12,22] no time-delays, or phase lags, are present on the experimental time scale either from the lifetime of the electronically excited states or from any subsequent vibrational excitation due to the translationally hot I atoms produced (see earlier discussion on vibrational-translational relaxation times in the presence of 50 Torr He). For a particular sample and excitation wave-
3. Experimental The optoacoustic cell used was similar to that described for other measurements [6,29], being machined from stainless steel with a volume of 10 cm3 and a pathlength of 3 cm. A special needlevalve plus bellows arrangement ensured a completely smooth inner surface when the cell was closed; no problems due to Helmholtz resonance were thus present. Extensive anti-vibration measures were taken.
VACWM SV%TEM
My Ar+ LASER
b EA
Fig. 1. splitter, position ulator,
Simplified experimental arrangement. BS is beam M is high reflectivity mirror, MM is movable doublemirror, PM is photomultiplier, IM is intensity modand EA is electronic analysers of microphone signal.
T. F. Hunter,
C. M. Leong / Absolute .vields of I(‘P,j2)
length, consecutive measurements were carried out using Ar+ and dye laser excitation by suitable positioning of the movable mirror MM in fig. 1. In fig. 1, all mirrors are high reflectance (> 99%) and the beam splitter, BS, gave 80-90% transmittance between 570 and 620 nm and = 98% reflectance at 514-450 nm, both for 45O incidence. Any small drift in phase measurement over an extended period of time was corrected for by determining the phase of the modulation of the laser beam using a photo-multiplier as shown in fig. 1.
149
in I> photodissociation
30
__I-i-i--1--_,
- I.
-1.1
_-I.,” %
V
o
F
.I.
-‘=”
207y+ , 10
,
0
,o’
0
/O’
0
SC
o--o-o
-
/O’
‘0,
‘0.
c
-0.
-0
,
100
200 Time / mm
Fig. 3. Examples of plots of the relaxation phase lag, 0,. as a function of time for excitation by Ar+ laser lines at (a) 476.5 nm, (b) 501.7 nm, and (c) 514.5 nm.
4. Results and discussion With phase lag measured in a clockwise direction, from the experimental procedure given in section 3, the desired relaxation phase lag 6, is given by
where 19,~ and eMP refer to phases measured on the photomultiplier and microphone respectively. As shown in eq. (10) the quantity of interest in the measurement of q is (e,,,),,, and figs. 2 and 3 show examples of typical plots of 0, against time of run. This is equivalent to t9, versus pressure of I, but the time axis is not linear in pressure; this is
of no consequence since it is only the value of e,,, at its maximum which is of interest. An example of the way in which the pressure of I, falls in the cell once the cell is closed is shown in fig. 4, for some typical dosing; the I, relative pressure is taken from the amplitude of the dye laser acoustic signal. Fig. 2 shows values of 0, for 488 nm excitation, indicating the range of curves for different dosing characteristics. By “different dosing characteristics” is meant the following. If a pressure of I, is left in the cell overnight prior to the
I , 6-
01 0
I
1
1
100
200
300
Time/ min
Fig. 2. Plots of the relaxation phase lag, t$,,, as a function of time after the cell is closed, for excitation by Ar? laser line at 488.0 mn. The different plots represent different dosing characteristics by I, prior to the measurement. The maximum 0, is the same, within experimental error, in each case.
Time /hours
Fig. 4. Typical fall of dye laser induced acoustic signal amplitude (arbitrary units) as a function of time. The signal amplitude is directly proportional to I, pressure in the cell.
150
T. F. Huplter, C.M. Leo~tg /
Absoluteyields of I(‘P,,,)
experiment then the cell walls are well covered and the rate of loss of I, to the walls in the actual experiment is slow. If the cell has been strongly evacuated prior to the experiment then I, adsorption will be much faster. In table 1 are given (e,,,),, values for the various wavelengths, the value at 457.9 nm being less accurate because the various I, dosing characteristics tried did not give so clear a maximum as for the other excitation wavelengths. Another method of measurement is exemplified by results taken at 496.5 nm excitation and shown in fig. 5. From eq. (9) it can be seen that a plot of ( p tan 8,))’ versus p-’ should be linear and that from the subsequent intercept and gradient (H + K)/L is available. The relative I, pressure is sufficient in this analysis and plot (fig. 5) and such pressures are measured from the dye laser induced acoustic amplitude; p is therefore in arbitrary units. The resultant q values in table 1 come from the listed H/L data using eqs. (10) and (5) and the (hv - 0) values given in the table. K has a constant value of 306 cm-’ in each case. The q values are also plotted in fig. 6 for comparison with previous work, and it is obvious that the present optoacoustic measurements give results in essential agreement with other work involving determinations of I and I* concentrations. The minimum value of q for I atom lasing to occur is 0.67 (due to the degeneracy of the 2P1,2 and 2P3,2 states and the production of two I atoms from I*) and, within experimental error, this value is achieved at both 496.5 and 501.7 nm. Lasing is actually observed [2] from 493 to 501 nm, and it is
Table 1 Results for the various parameters, see text x (nm)
(k),, (de&
457.9 416.5 488.0 496.5 501.7 514.5
7.5 12.5 23.0 nr 29.5 9.5
” (K + H)/L
is found
parameters
used in the calculation
(K + H)/L
1)~I, photo~issocirrtio,l
0
‘/P2 Fig. 5. Plot of (p tan 6,) ’ versusp -*. where p is I, pressure in arbitrary units and 0, is relaxation phase lag, for excitation with Ar+ laser line at 496.5 nm.
obvious that the required population of I* is only just obtained; the I atom laser from photolysis of I, is operating very much on threshold. It is useful at this stage to indicate briefly how q values were obtained from previous work and thus presented for comparison in fig. 6. Oldman et al. [12] used photofragment spectroscopy and distinguished clearly the extent of absorption from the X state to the B and lIIlu states (the A state does not absorb in the wavelength range covered by them). The measured ratios of ‘IIIIU + X to B + X, with the q value in parentheses, were 1.2 + 0.2 (0.46 k 0.04), 1.3 k 0.1 (0.44 -t 0.02) and 2.3 f 5 (0.31 f 0.05) at 479.6, 464.9 and 449.8 nm respectively. Comparison with the present values at 457.9 and 476.5 nm gives excellent agreement. Broadbent and Callaer [13] found the quantum yield of I* at four excitation wavelengths using the amount of alkyl iodide formation in mixtures
of 4, the quantum
H/L
yield
of I( ‘P t,*)
hu(cm
i 2.0 i_ 1.0 i_ 1.0 * 1.0 * 1.0
3.81k 1.20 1.83 *0.19 0.78 + 0.05 0.65 + 0.08 0.52 + 0.03 2.56 kO.32
from data in fig. 5.
40
20
3.66+ 1.02 1.73iO.18 0.72 i 0.05 0.5910.08 0.46 i 0.03 2.41 I 0.31
D
-I )
9399 8546 8052 7701 7492 6996
formation.
For definitions
Y 0.29 0.41 0.62 0.64 0.68 0.27
f f f + + f
0.06 0.03 0.02 0.03 0.02 0.03
of
T. F. Hunter, C. M. Lemg / A hsolute vi&Is
10
-I
+ 0,
I,
440
I
I
460
I,
I
I
I
I
I
480
I
500
I
,.
I.
0 1
520
h /nm
Fig. 6. Quantum field of I( 2P,,2) production, q, as a function of excitation wavelength, X. ( I) is present results, and the other references arc as follows: (X) [12], (0) [13] in He, (0) [32], (+) [15] and (A) [16]. The horizontal line represents the threshold for lasing.
of I,, propane and inert gas. The q values they obtained at the wavelengths 483.5, 502.0, 515.5 and 533.0 nm were respectively 0.49, 0.41, 0.28 and 0.08 for He as the inert gas, and 0.47, 0.47, 0.31 and 0.03 with Xe. The values at 483.5 and 502.0 nm are considerably lower than the present values, although the 515.5 nm value is within experimental error of that measured at 514.5 nm in this work. Tellinghuisen [14] produced careful I, absorption data in this wavelength region and from his values for ‘IIt” + X vis-a-vi, B +- X absorption strengths, values of q can be calculated with the assumption that the ‘IIlu state only produces 21 and the B state only I* plus I. Such values are shown in fig. 6 over the range 440 nm (q = 0.39) to 510 nm (q < 0.74). Both Burde et al. [15] and Wiessenfeld and Young [16] used pulsed tunable dye laser excitation and followed the extent of I or I* spectroscopically. These q values should be the most accurate of the previous data and should also be useful in the comparison of techniques mentioned in section 1, i.e. in measurements on alkyl iodides the optoacoustic results seem lower than those obtained by spectroscopic measurements on I and I * concentrations. The q values obtained are given in fig. 6, but, for more accurate comparison, the highest values measured by Burde et al. [15] were 0.63 and 0.63 at 495 and 500 nm. Wiesenfeld and Young [16] also got their highest values in this
of I( ‘P,,_,)
iu I2 photodissociution
151
region, namely 0.667 and 0.682 at 495 and 498 nm respectively. In both cases [15,16] the agreement is good with the optoacoustic data at 496.5 and 501.7 nm, well within the experimental error, and there seems no reason to suspect that the optoacoustic technique has an inherent tendency to produce low results. The conclusion is that on studies of photofragmentation in alkyl iodides the optoacoustic results are likely to be fairly accurate and that the high values obtained by I atom spectroscopic techniques (and the value of unity for q often assumed for n-C,F,I photolysis) are inaccurate for some reason; see section 1. Such lower values for q for alkyl and fluoro-alkyl iodides have significant modelling implications for the development of iodide lasers pumped by radiation from the sun and of possible application in space. I(‘P,,,) formation at energies below the B state dissociation limit. The dissociation limit of the B state is at a wavenumber of 20043 m- ‘. Two of the wavelengths used in this study, 501.7 nm (19932 cm-‘) and 514.5 nm (19436 cm-‘) are thus at energies where extra energy is required to explain the q values reported. Other workers [13,15,16,31,32] have made similar observations. It has been suggested [33] that production of I(2Pr,2) below the dissociation limit may be, at least in part, due to B + X absorption from “hot bands”, i.e. some of the absorption is from thermally excited rotational and rovibrational levels of the X state. This is certainly possible for the results of Callear et al. [13,31,32], who carried out the main study below the dissociation limit, where continuous radiation with interference filters was used. However, in the present case the details of absorption from the two laser lines at 501.7 and 514.5 nm have been studied [34-361. With 501.7 nm excitation a number of B + X rovibrational transitions are possible [35], all originating in u” = 0, and more than 90% of the structured absorption (i.e. to the B state) is due to 62-0, R26 [34,35]. The extra energy, E, required to reach the dissociation limit with 501.7 nm excitation is 111 cm-‘. Using, for I*(X), values [37] of the rotational constant of 3.737 X lo-* cm-’ and for the centrifugal distortion constant of 4.25 x lop9 cm-‘, the dominant transition only provides an
152
T. F. Hunter, C. M. Lemg / Absoluie~vields of I( ‘P,/_,) 1~ I_, photodssociutim
extra 26 cm-‘, and other sources, presumably collisional effects on I,(B state), must be involved. For 514.5 nm excitation e is 607 cm- ’ and [34,36] more than 90% of the absorption is in 43-0, P13 and R15. These two respectively only contribute 7 and, although the small extent of and 9 cm-’ other transitions may be more important, any contribution from thermally populated levels is obviously very much less than 607 cm-‘. It therefore appears certain that, in the present measurements at 501.7 and 514.5 nm, no contribution to the measured q comes from direct dissociation. In the region of these two wavelengths, and with the collisional effect of 50 Torr He present, previous work [38-401 shows that fluorescence is negligible; the only two processes of importance are I,(B) + 21 in spontaneous and collisional predissociation and I,(B) -+ I + I * in some collisional release mechanism. The initial levels excited are B, u’ = 62 with 501.7 nm excitation and B, u’ = 43 with 514.5 nm excitation, and the He collisions may spread this energy to close-lying vibrational levels before the above important dissociative channels operate [40]. The measured extent of I* formation is 0.68 and 0.27 at 501.7 and 514.5 nm respectively. The smoothed data of Tellinghuisen [33] give the fractional absorption to the B state at 0.67 and 0.77 at these two wavelengths, which yields for the quantum efficiency, say F, from the B state alone the values of 1.01 and 0.35 at 501.7 and 514.5 nm. The relative contributions of ‘IIlu + X and B + X given by Tellinghuisen [33] may not be very accurate in the banded region for specific laser lines, but simple extrapolation from the continuous region suggests a substantial presence of the ‘Hi,, + X absorption. Other authors, viz. Burde et al. [15] using high-resolution dye laser excitation, have accepted the Tellinghuisen data in this wavelength region. Because of the nature of the present excitation the high F values are obviously due to excited state collisions. Although produced, or selected, in a cool rotational state, the 50 Torr He ensures that rotational thermalisation of the excited I, molecules will occur. In the work of Collear et al. [13] high pressures of added gas were also present; their result at 515.5 nm is very close to that
measured here at 514.5 nm although at a similar wavelength to 501.7 nm their result is lower by around a factor of 0.7 (see fig. 7). The suggestion [41] that complex formation between the excited I, molecule and the additive contributed available energy to the I, bond rupture (thus making I* formation possible) was discounted when Broadbent and Collear [13] found similar efficiencies for a variety of additives, viz. He, Xe, C,H,, H,, N,, Ar and CO,. The remaining possibility that the physical degrees of freedom in the I,(B) + M interaction contribute efficiently in producing I* is thus a very likely explanation. Considering the relative translation, the I, internal rotation and the orbited rotation of I, relative to M [13], the probability P that energy greater than E is available in the system is given by P = [t(~/kT)~
+ c/kT+
l] exp( -r/kT).
(11)
P represents a maximum since all such energy could not be expected to contribute to release of I*; nevertheless in fig. 7, where the solid plot is of P, the results at 501.7 and 514.5 nm are seen to give good agreement. Burde et al. [15], using low pressures of I, alone, found values below both the P plot and the present values (fig. 7). Since I*(X)
0,4@2-
W&V, 500
I 510
I 520
. I
I: I 530
h/m
Fig. 7. F values for excitation of I, at energies below the I l(B state) dissociation limit. F is quantum yield of I( 2P1,2) formation following B state excitation. ($) is present result, (0) [15], (x) [13] in He, (+) [13] in Xe. The solid line is calculated using eq. (11).
T. F. Hunter. C. M. Leong / Absolute .vields of I(2P,,2)
collisions are known to have essentially unit efficiency [42] in producing either 21 or I + I*, in experiments with I, alone present, no rotational thermalisation of I,(B) will take place. It would appear therefore that the dye laser excitation of Burde et al. [ 151 must produce I,(B) in a cooled rotational state, or, if rotational energy is not the important factor, the propensity for producing I* must be less in It(X) than in He collisions, i.e. the branching ratio ( *PI,* )/( *P3,* ) in I*(B) + M + I (’ P3,2 ) + (I(2p3,2) orI(2P1,2))
+ M
is higher for the inert gas. Neither the calculated P factor of eq. (11) nor arguments based on the polarizability of M fit in with this. It is obvious that the extent of I(*P,,,) formation at wavelengths below the B state dissociation limit requires a rather more careful experimental and theoretical investigation.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15]
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1161J.R. Wiesenfeld