doppler line shape of atomic fluorescence of sodium iodide
TRANSCRIPT
Doppler Line Shape of Atomic Fluorescence of Sodium IodideHoward G. Hanson Citation: The Journal of Chemical Physics 47, 4773 (1967); doi: 10.1063/1.1701696 View online: http://dx.doi.org/10.1063/1.1701696 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/47/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Refractive index of sodium iodide J. Appl. Phys. 111, 043521 (2012); 10.1063/1.3689746 Super-radiance in the sodium resonance lines from sodium iodide arc lamps Appl. Phys. Lett. 97, 061501 (2010); 10.1063/1.3479522 Line shape analysis of Doppler broadened frequencymodulated line spectra J. Chem. Phys. 104, 2129 (1996); 10.1063/1.470969 Efficiencies of Sodium Iodide Crystals Rev. Sci. Instrum. 29, 406 (1958); 10.1063/1.1716209 Fluorescence Intensity Ratio of Sodium Doublet Observed in the Optical Dissociation of Sodium IodideVapor J. Chem. Phys. 27, 491 (1957); 10.1063/1.1743754
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Sat, 20 Dec 2014 13:22:15
THE JOURNAL OF CHEMICAL PHYSICS VOLUME 47, NUMBER 11 1 DECEMBER 1967
Doppler Line Shape of Atomic Fluorescence of Sodium Iodide*
HOWARD G. HA.'!soN
University of Minnesota, Duluth, Dulttth, Minnesota
(Received 31 May 1967)
Measurements on the Doppler profiles of the sodium D line ator;tic fluores<;ence ?f s~dium iodide which accompanies dissociation of the molecule in the vapor state by polanzed ultraviolet light 10 t~e 2100-2500-A range show no pronounced directional effects. The known hyperfine structure of the sodIUm states, the response of the measuring instrument, and the ~xpe~~ental uncert~inties are tak~n int~ account. It is suggested that the upper repulsive state of the diSSOCiating molecule IS n~t necessanly a s10gle state, and that some of these upper states possess minima in which some of the eXCited mol~cules are trapped f.or a short time. The measured Doppler speeds are somewhat less than expected when higher energy ultravIOlet photons are used, which may be due to dissociations in which the iodine atom is also excited.
INTRODUCTION
The atomic fluorescence of sodium iodide has been investigated by many workers.l-7 The Do~pler br~adening of the resulting D lines from the eXCIted sodIUm atoms was first demonstrated by resonance absorption methods by Rogness and Franck.3 The de~e?-dence of intensity of atomic fluorescence on the eXCltmg wav~length has been previously I?easured ~nd an a~proxImation of the upper repulSIve state mvolved m the dissociation of the sodium iodide molecule has been deduced using the Franck-Condon principle.6
Models of the spatial characteristics of molecular dissociation based on varying assumptions concerning the details of the photodissociative encounter betwe~n a polarized ultraviolet photon and the molecule In
certain vibrational and rotational states have been described by Zare and Herschbach.8 They have calculated for several sets of assumptions, the Doppler line form' factors to be expected for atomic fluorescence excited by polarized ultraviolet radiatior:. E~pli.cit curves have been given8 for the case of sodIUm IOdIde based on the excitation-curve data.6 These predicted Doppler line curves have been calculated using parameters averaged over vibrational and rotation states of the dissociating molecule and also averaged over. the thermal and dissociative kinetic energies of the eXCIted sodium atoms.
In this paper the results of experimental measurements of the Doppler line profiles of the sodium D lines resulting from atomic fluorescence are presented.
EXPERIMENTAL
Powdered sodium iodide salt was placed in a 13-mm diameter, thin-walled, Suprasil fused-quartz tube which
* Partially supported by the U.S. Office of Naval Research under contract No. Nonr 710(59).
1 A. Terenin, Z. Physik 37, 98 (1926). . 2 T. R. Hogness and J. Franck, Z. Physik 44, 26 (1927). 3 J. G. Winans, Z. Physi)r, 60, .631 (1930). . • A. Terenin and N. PrileshaJews, Z. Physik. Chern. Bl3, 72
(1931). . k Ph 'k Z 6 B. Kisilbach, V. Kondrat jew, and A. Lelpuns y, YSI. .
Sowjetunion 2, 201 (1932). • H. G. Hanson, J. Chern. Phys. 23, 1391 (1955). 7 H. G. Hanson, J. Chern. Phys. 27, 491 (1957). 8 R. N. Zare and D. R. Herschbach, Proc. IEEE 51,183 (1963).
was then placed concentric with a thin-walled steel cylindrical tube which was 18 cm long and 4.5 cm in diameter. The steel tube was heated by resistive heating by currents furnished by a 2.5-kVA transformer with variac control. Two transverse sets of holes of 6-mm diameter were drilled in the middle of the cylindrical heater with their axes perpendicular to each other and the axis of the cylinder. The exciting ultraviolet light was focused through one of these sets of holes and the resulting fluorescence was observed through the other set.
The salt in the fused-quartz tube was kept from subliming too rapidly by placing a glass-wool plug in the open upper end of the tube. The ends of the tube were the coolest parts so that salt which sublimed from the salt supply surface condensed on the glass wool and formed a porous plug which prevented further rapid sublimation of the salt.
The heating tube and salt tube were in a large cylindrical steel vacuum chamber furnished with Suprasil fused-quartz windows. The vacuum achievable with the heated salt in place was 2 X 10-6 mm Hg. The temperature of the heated salt was measured with a thermocouple wound around the fused quartz tube so t~at its sensing end was at the level of the condensmg surface of the porous salt plug. Temperatures of 500°-600°C were used.
Various ultraviolet sources were used, including condensed sparks (Zn, Cd, AI, Fe, and Ag electrodes) and a hydrogen arc. The hydrogen arc was the Ranovia water-cooled type operating at about 0.4 A from a 5000-V transformer. The ultraviolet light was either focused directly through the vacuum chamber window by a Suprasil fused-quartz lens on to the heated salt vapor or was first passed through a Bausch & Lomb high-intensity grating monochromator to isolate certain exciting wavelengths. Provisions were made for placing polarizing plates9 in the ultraviolet beam. 0
The polarizing plates were tested from 2000 to 2500 A for percentage polarization and transmission of light. Two monochromators were used in series to reduce
9 Polacoat Inc., 9750 Conklin Rd., Blue Ash, Ohio.
4773
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Sat, 20 Dec 2014 13:22:15
4774 HOWARD G. HANSON
the scattered light and slit bandpasses were about 40 A. The percent polarization was better than 96% for all settings in the range even though the collimation of the beam was not critically adjusted. The raw transmission through a single polarizer changed smoothly from 4.0% to 19.7% in the same range while the transmission through a second analyzing plate placed in the beam at maximum transmission changed correspondingly from 6.9% to 37.5%.
The single-etalon system was built similar to a single element of the multi-etalon system described by Mack.lO
The spacing of the 45-mm diameter Hilger and Watts silvered etalons was such that the free spectral range was 0.800 cm-I to accommodate the expected Doppler widths and was so chosen that the two D lines did not have overlapping orders. The apertures were 1.5 mm in diameter in order to allow sufficient flux through the system. The telescope and collimator lenses were 2-in. diameter achromats of about 30-cm focal length. The whole system was mounted on a heavy steel beam. The air-pressure scanning was achieved by pumping out the vacuum cylinder that contained the etalons and allowed air to leak back into the system through a needle leak valve and flow gage. A pressure transducer produced a dc signal proportional to the pressure in the etalon chamber.
The light passing through the exit aperture in the etalon system was measured by a 9558 QA EMI tube with 8-20 response which could be cooled by dry ice. The signal from the photomultiplier was amplified and applied to one axis on an X - Y recorder while the other axis was driven by the signal from the pressure transducer.
The response of the system was calibrated before and after each run using a discharge tube operating at room temperature and containing Hg 198. This source gave simple narrow line profiles, free from hyperfine structure, whose total measured widths were essentially a measure of the response of the instrument. The instrument response width at half-intensity height of these narrow line profiles depended on the size of the apertures and the over-all finesse of the etalon system. Under the conditions necessary for measuring the fluorescence, values near 0.135 cm-I were obtained for these half-height widths; which indicates it was possible to detect significant change in widths, depending on the steadiness of the source, of from 0.010 to 0.030 cm-I • The measured half-intensity widthslO were used as parameters in developing an Airy function description of the response of the etalon system.
A narrow band-pass interference filterll with a halftransmission height width of 2-3 A was used to isolate the two D lines in the process of calibrating the etalon and in making some of the measurements.
The line shape, as recorded on the X - Y recorder, depends on three factors (a) the resolution of the
10 J. E. Mack et al., Appl. Opt. 2,873 (1963). 11 Spectrolab; Sylmar, California.
etalon system, (b) the fine structure of the sodium atomic states, and (c) the Doppler effect. In order to evaluate (c), allowance must be made for (a) and (b). The hyperfine structure separationsl2 of the atomic states of 23N a involved in the D-line transitions (up to 0.059 cm-I in magnitude) were used together with the measured Airy response curves of the etalon system to make calibration curves by the use of a computer program. Doppler profiles (corresponding to Maxwellian distributions, or corresponding to distributions as deduced by Zare and Herschbach, or by deductions based on Ref. 6) of the composite line made up by the sum of the overlapping fine structure components, were scanned by the computer as they would be seen by an instrument whose response curve was that of the experimentally known Airy response curve. By varying the width and distribution of the Doppler profiles fed into the computer program, a set of calibration curves whose parameters were known could be compared with the actual spectra.
A comparison experiment on measuring the Doppler width of the sodium D lines from a sodium resonance lamp gave agreement with expected values within the uncertainties caused by the easily observed self-reversal of the lines. Measurements on the light from a cool sodium lamp with a freshly struck arc gave the best agreement between calculated and observed linewidths.
RESULTS AND INTERPRETATION
The form factors discussed by Zare and Herschbach in Table V, Ref. 8 are shown for convenience in Table 1. P2F is the factor H3vF2-V2)/V2, where v is the speed of an emitting atom and VF is the component of the velocity of the atom in the direction of observation.
The form factors, the dissociative speed spectrum, the hyperfine structure of the fluorescence line, and the resolution of the instrument scanning the fluorescence line will all be of importance in determining the profile of the observed fluorescence. The hyperfine spacing and the resolution are known and account has been taken of them. The dissociative speeds can be varied by choosing different ultraviolet-excitation sources. The appropriatness of a given form factor can then be judged by comparing the actual profiles measured in different directions of observation with the profiles computed for the cases shown in Table 1.
A Cd spark source was chosen to excite the atomic fluorescence because the strong lines at 2144, 2195, 2265, 2288, and 2313 A give a high intensity of fluorescence, are polarizable, and lie in a range such that the expected dissociative speeds can be evaluated from Ref. 6. A further advantage is that no Cd spark line lies near the long-wavelength limit for exciting the fluorescence. This avoids giving rise to low dissociative speeds whose Doppler contributions would be minimum.
Observations were made in two directions: parallel
12 P. Kusch and V. W. Hughes, Handbuch der Physik (SpcingerVerlag, Berlin, 1959), Vol. 37, p. 100.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Sat, 20 Dec 2014 13:22:15
FLUORESCENCE OF SODIUM IODIDE 4775
TABLE 1. Form factors for Doppler broadening of atomic fluorescence line excited by polarized uv (adapted from Zare and Herschbach) .•
Electronic Observation Axial Transverse transition direction recoil recoil
Case A II Z (1) 1+2P2F (4) 1-P.,
Case BI l. Z (4) 1-P2F (2) 1+!P2F
Case B211 Y (4) 1-P2F (2) H!P2F
Case C l. Y (2) 1+!P2F (3) 1-iP2F
• The notation is that of Zare and Herschbach. Ref. 8. The electric vector of the uv light is parallel to the Z axis. The exciting beam is in the X direction. Each form factor is to be mnltiplied by the isotropic form factor R. = 1/ •. The numbers in parentheses indicate the curves in Fig. 2.
to the electric vector of the polarized light (Z) and perpendicular to the electric vector (Y). All observations were made in a direction perpendicular to the beam of exciting ultraviolet light (X).
The most significant result of the line profile measurements was the lack oj experimentally observable difference between the profiles observed Jrom the Y and Z directions. Figure 1 shows four experimental profile curves superimposed on each other (two in the Z direction and two in the Y direction). Within experimental limits the profiles appear identical. This lack of any observed difference ascribable to preferred dissociative directionality is in agreement with the results reported by Mitchell,13 and is compatible with the possible presence of mixed bundles of upper molecular states as discussed by Zare and Herschbach.14 Electron impact ex-
(f) ~
Z
10
8
::J 6
>-0:: <r 0:: 4 ~
iIi 0:: <r 2
oL-----.1±o-----.~2~0----~.3~0-----A+.0~---.~5~0-
CM- I
FIG. 1. Doppler profile of atomic fluorescence of sodium iodide dissociated by polarized ultraviolet light. The heavy lines represent viewing in the Z direction (perpendicular to the electric vector). The dashed lines represent viewing in the Y direction (parallel to the electric vector). Within an uncertainty of less than 0.030 cm-I there is no difference in the widths at half-height which depends on the direction of viewing. All other curves fell between the extremes shown.
13 A. C. G. Mitchell, Z. Physik 49, 228 (1928). 14 R. N. Zare and D. R. Herschbach, J. Mol. Spectry. 15, 462
(1965) .
10
8 (f) ~ Z ::J 6 >-0:: <r 0:: ~
4
iIi 0:: <r 2
0 .10 .20 .30 .40 .50
CM -I
FIG. 2. The computed line profiles of atomic fluorescence of sodium iodide taking into account hyperfine structure, speed distribution, adjacent doublet line, and instrument response. The numbers refer to the cases in Table 1.
periments on N2 by Kieffer and Van Brunt15 also show very little directional preference for the ions formed in the dissociation process.
To check the experimental tolerances necessary for the ability to detect differences in line profiles, which are observed from the Y and Z directions for the different cases in Table I, it is necessary to evaluate the combined masking effect of (a) the presence of hyperfine structure and (b) the use of a relatively low-resolution measuring instrument. Figure 2 shows a set of computed profiles for making such comparisons among the four separate form factors in Table 1.
Each computed profile is for a single Doppler speed Vav of 1.4 X lOS cm/ sec and a different form factor (and includes the hyperfine structure and effects of resolution, but ignores the small correction for thermal motion). As can be seen from Fig. 3, Vav is a good approxi-
w > Si -.J W 0::
10
2
V (CM/SEC X 10 5)
FIG. 3. The speed distribution of excited sodium atoms resulting from the photodissociation of NaI when the exciting source is a cadmium spark. Deduced from data in Ref. 6. The short bold lines represent the most probable speed corresponding to each of the five cadmium-spark lines.
,. L. J. Kieffer and R. J. Van Brunt, J. Chem. Phys. 46, 2728 (1967) .
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Sat, 20 Dec 2014 13:22:15
4776 HOWARD G. HANSON
TABLE II. Results.
Measured Doppler Wl/2 width Modal v
Exciting source (cm-I ) (cm-I ) (cm/sec)
Cd spark 0.257 0.142 1.38X1Q6
Cd-spark lines isolated by monochromator 0.240 0.123 1. 20X 10·
Zn spark 0.238 0.120 1.17X10'
Zn-spark lines isolated by monochromator 0.268 0.152 1. 48X1Q6
mation to the expected v distribution when excitation is by Cd spark. The expected speed distribution shown in Fig. 3 for Cd-spark excitations was deduced from the data in Fig. 6, Ref. 6. By noting the energy of each Cd-spark line and taking account of the amplitude of the probability density loops for each vibrational level it is possible to calculate the expected v values corresponding to each contributing line. A separate measurement of the relative intensities of the spark lines using a monochromator with a sodium salicylate plate and a photomultiplier then enabled the calculation of the relative statistical weight for each v value.
From Table I and Fig. 2 one can deduce, for each case corresponding to an unmixed upper state, the maximum line profile difference expected when observation is changed from the Y to the Z direction. This profile difference can be expressed in terms of the difference between the values of W l / 2 (width at halfheight of the profile) when viewing along Y and the value of W l / 2 when viewing along Z. If the coupling in the sodium iodide atom is such that the dipole moment is parallel to the internuclear axis we must compare Cases A and B2 for axial and transverse recoil, and similarly if the dipole moment is perpendicular to the internuclear axis, we compare BI and C for axial and transverse recoil. The expected difference in W l /2 for these cases are: dipole parallel: (A vs B2) ax = 0.112, (A vs B2)tr=0.OSl; and dipole perpendicular: (Bl vs C)ax=O.OSl, (Bl vs C)tr=0.027 (all in em-I).
These expected differences in W l /2 for the various assumed cases must be compared to the experimental uncertainties. For the seven line profiles (three along Y, four along Z) taken under the most favorable experimental conditions of spark stability, intensity, salt vapor pressure, etc., the extreme scatter in the values of W l / 2 did not exceed 0.022 Cln-I and there was no discernible dependence on whether the viewing was along the Y or along the Z axis. If one assumes no mixture of statesl6 for the upper potential curve and recalls that a difference of 0.030 cm-l in the W l / 2 values would be detectable, the cases corresponding to the first three W l /2 differences above would have to be ruled out. The last comparison (Bl vs C)tr=0.027 cm-I is marginal, but it seems unlikely that this difference
l' Reference 8, p. 180.
in W l /2 would go undetected if the states were not mixed.
However, if one considers the upper potential curve to represent a mixture of states/3•lS the experimental evidence indicates a lack of anisotropy brought about by the mixing of ionic and atomic binding characteristics of the molecular states or the presence of other states from which dissociation and atomic fluorescence can occur. Calculations by Zare and HerschbachI4 indio cate that relatively sm~ll percentages of admixture can obliterate the directional effects.
Further evidence that mixed upper states are involved in the dissociation process is the experimental observation7 that the D2/ Dl intensity ratio of the sodium doublet varies with the wavelength of the exciting ultraviolet photon. The relative number of molecules dissociating with the sodium atoms in the P3/2 or P l/2 state apparently depends on the accessability of the upper states for various exciting energies in terms of the Franck-Condon principle.
A proposed explanation of both the narrowness of the Doppler width and the lack of observational directionality involves the presence of a higher excited molecular state. A possible state might be one which dissociates according to
NaI +h~Na(2Pl/2.3/2) + I (2Pl/2) , (1)
which requires 7600 cm-l more energy for the iodine excitation than the dissociation
NaI +h~Na(2Pl/2.3/2) + I (2P3/2) , (2)
for which evidence from absorption studies has recently been given by Davidovits and BrodheadP Their study supports the suggestionS that a minimum exists in the upper curve corresponding to (2).
The molecular state implied by (1) above would require the total energy of the shortest wavelength photons used in this study, and even then transitions upward would occur only from the highest vibrational levels of the ground state unless the upper curve possesses a minimum. If the minimum in the upper curve exists, dissociations would have to occur by collisional activation or rotational predissociation for those excited molecules for which the upward transition did not fall above the asymptote to the upper curve. Any existing directionality from such dissociations would be unobservable because of the small kinetic energies of separation for the directly dissociating molecules and because of the loss of orientation by the molecules experiencing delayed dissociation. Transitions from the upper state of (1) to the state implied by (2) with subsequent dissociation would also destroy the possibility of observing directionality. The low kinetic energies of separation involved would, of course, explain the narrowness of the Doppler widths observed.
To evaluate the Doppler widths, shown in Table II,
17 P. Davidovits and D. R. Brodhead, J. Chern. Phys. 46, 2968 (1967) .
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Sat, 20 Dec 2014 13:22:15
FLUORESCENCE OF SODIUM IODIDE 4777
calibration line profiles taking into account known hyperfine structure, known etalon response, and using one of two assumed speed distributions (Le., Maxwellian or the narrow type of Fig. 3) were compared with the measured line profiles. The largest effective width at half-height due to the Doppler component was found to be 0.152 cm-l (W1/ 2=0.268 em-I) for a Zn spark as source through a monochromator which isolated the strong line group near 2100 1. This value of 0.152 cm-l corresponds to a modal speed of 1.48X1OS em/sec and a kinetic energy in agreement with a point only 1670 em-l above the asymptote in Fig. 5, Ref. 6. Even if the asymptote were shifted upward a maximum allowable amount, the calculated excess kinetic energy would be expected to correspond to at least 5400 cm-1
when excitation is in the 2026-2139-1 region. As proposed, a mechanism for accounting for this energy discrepancy might be through the presence of a higher upper state with a minimum from whence some of the excited molecules could suffer dissociations yielding lesser amounts of kinetic energy.
The relatively low Doppler width, measured for the fluorescence generated by the Zn spark used without the monochromator, is attributed to the effect of a strong continuum in the spark, as operated, which begins at 2300 A and continues to longer wavelengths and attaches to the strong Zn line at 2502 1. These wavelengths would generate low speed dissociating atoms.
The cadmium-line group, when isolated by the monochromator and used as exciting source, gave a measured value of modal velocity of 1.2X lOS em/sec corresponding to a kinetic-energy excess above the asymptote of only 1090 cm-1 compared to an expected value of at least 1500 em-I, even when allowing for a shift of 330 cm-l for the asymptote to the curve along which dissociation is assumed to take place.
The Cd-spark value, when used without the monochromator, of 1.38X1OS em/sec is somewhat high. In the absence of a monochromator, the Suprasil lens
system would transmit the four strong lines at 1844-1901 1 which might well give rise to transitions from which dissociations of high velocity are possible without transitions to an intervening state.
Another indication that a shallow minimum exists in the potential curve along which the sodium and iodine atom dissociates is the previously reported observation6 that when a nonquenching gas is introduced into the fluorescence chamber an enhancement of 25% is observed. This has variously been ascribed to redistribution of the salt vaporS or to depolarizationl8 of the D-line fluorescence itself. However, the enhancement might equally well be attributed to collisionactivated dissociations of the sodium iodide molecule from the upper state with the shallow minimum. At the temperature used, the mean translational kinetic energy corresponds to nearly 900 em-I, so relaxation into vibrational equilibrium appropriate to this temperature by collision processes with the foreign gas would involve a substantial number of transitions to unstable vibrational or rotational states (above the asymptote) from which dissociation of the excited sodium iodide molecule would occur.
An experimental measurement of the effective mean lifetime of the fluorescence when excited by various wavelengths of ultraviolet radiation would make it possible to determine the delay brought about by the proposed intermediate upper states or by the brief retention in the suggested shallow minimum in the potential curve along which dissociation occurs. These measurements, together with further measurements of the Doppler broadening, are being undertaken.
ACKNOWLEDGMENTS
The author wishes to thank Michael Sydor for helpful discussions of this work and the Graduate School of the University of Minnesota for equipment grants.
18 J. G. Winans and E. J. Selden, Handbook of Physics (McGrawHill Book Co., New York, 1958), Pt. 6, Chap. 7, p. 132.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Sat, 20 Dec 2014 13:22:15