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High-resolution saturation spectroscopy of the H2n = 3 triplet states
Volume 190, number 3,4
CHEMICALPHYSICSLETTERS
6 March 1992
High-resolution saturation spectroscopy of the H2 n = 3 triplet states L. Jozefowski, Ch. Ottinger a n d T. R o x Max-Planck-lnstitut far StrOmungsforschung, Bunsenstrasse 10, W-3400 G6ttingen, Germany
Received 25 October 1991
For measurementsof the fine and hyperfinestructure of the H2 n = 3 complexof triplet states, laser saturation spectroscopyhas been applied to a beam of metastable H 2 molecules. A spectral resolution of better than 20 MHz (fwhm) was achieved. The experimental arrangement is described in detail, and preliminary results are given.
The electronic structure of highly excited Rydberg states of H2 m'olecules is of fundamental interest. It can be elucidated by means of high-resolution laser spectroscopy. In particular, the four strongly mixed triplet states with n = 3 , (3s)g3Z~, (3d)h 3Z~-, (3d)i 3FI~ and (3d)j 3A~ which lie at Te~, 13.8 eV, are accessible from the metastable c 31-I~- state by absorption of visible laser light. This offers the possibility of determining accurately the absolute excitation energies as well as the fine and hyperfine structure of specific rovibrational levels. The metastable character of the c state itself was discovered by Lichten [ 1 ] in 1960 who also measured its fine and hyperfine structure by means of the molecular beam magnetic resonance technique [ 2 ]. Since then, there have been various laser spectroscopic investigations on beams of H 2 (C). The fine structure of the higherlying n = 3 triplet states was measured for some levels (v' = 0 and 1, N' = 2) of para-H2 by Lichten's group [3,4]. They used a sub-Doppler resolution technique based on the partial depletion of the H2 (c) beam by laser optical pumping. Eyler and Pipkin determined the radiative lifetime of the n = 3 and 4 states by means of laser-induced fluorescence (LIF) [5,6]. In both studies the same type of molecular beam apparatus was used, where HE molecules are excited by electron impact to the metastable c state. Bjerre and Helm [ 7 ] partially resolved the fine and hyperfine structure of two rotational lines of the HE(g~c, v " = 2 ) transition using the fast neutral
beam photofragment spectroscopy. Here H 2 (C) molecules are generated by neutralization of H~-. In the same manner Koot et al. [8 ] determined experimental wavefunctions of the excited states and analyzed the dissociation dynamics. High-resolution laser spectroscopy on molecular beams is normally done using narrow slits which collimate the molecular beam in the direction of the laser light, so that the Doppler broadening is minimized. For example, Geisen et al. [ 9 ] obtained a resolution of 8 MHz, mainly determined by the residual Doppler width, in measurements of the hyperfine structure of the B3FIg and A 3E ,+ states of molecular nitrogen. Lichten et al. [ 3 ], using the same method in their spectroscopy of the n = 3 triplet states of H2, reached a minimum Doppler width of about 16 MHz, additional to the natural linewidth and the power broadening. Another well-known method of achieving very high, Doppler-free resolution in laser spectroscopy is the saturation technique, with its variants of absorption, intermodulated fluorescence or polarization detection [ 10 ]. This technique is most often used in cell experiments, but it also offers the high resolution power in combination with molecular beams, as has been reported earlier [ 11 ]. With this method there is no need for any beam collimation, because the saturation signal automatically selects all those molecules whose velocity is precisely perpendicular to the laser beam. This is true even in the case of a wide molecular beam issuing from a large
0009-2614/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
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CHEMICALPHYSICSLETTERS
volume. The entire cross section of such a beam will contribute to the saturation signal. It is in this situation that the saturation technique is greatly superior to the collimation method. The latter would require two slits, both much narrower than the extended beam source, and would thus utilize only a small fraction of the total beam intensity. The present experiment is of the extended, diffuse source variety, therefore the saturation technique is very advantageous in providing a Doppler-free signal with a much larger intensity. An overview of the whole experimental arrangement of the present experiment is given in fig. l, while fig. 2 shows a perspective drawing of the parts essential to the saturation technique. The apparatus consists of two vacuum chambers made of stainless nonmagnetic steel. The nozzle chamber is pumped
Dye Laser
oHt!,
NoS c _l A1
A2
'
Fig. 1. Schematic overview of the experiment. A: anode; A 1, A2: apertures; A/D: amplifier and discriminator; C: cathode; Ch: chopper; FO: fiber optics; FPI: Fabry-Perot interferometer; G: gate; M: mirror; N,S: magnet; No: nozzle; PM: photomultiplier; PoM: powermeter; Sk: skimmer.
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6 March 1992
by an oil diffusion pump with a water cooled baffle, having an overall pumping speed of 3000 ~/s. The H2 background pressure in the chambers is about 5 × 10 -4 mbar. The main chamber is pumped by an oil diffusion pump with a freon cooled baffle, giving a pumping speed of 1500 I~/s. The pressure here is about 5 × 10 -6 mbar. Both pumps are backed by a 500 m3/h Roots and a 180 ma/h rotary pump. For ortho-H2 one can observe both fine and hyperfine structure, while for para-H2, which has zero total nuclear spin, only fine structure occurs. The possibility of preparing alternatively natural and para-H2 enriched gas proved to be a valuable aid in the analysis of the spectra. The natural hydrogen (99.99% purity) was used as supplied in a 200 bar cylinder, with a regulator valve providing the inlet pressure of 2.5 bar. The para-H2 enriched hydrogen was generated from liquid hydrogen by passing the evaporating gas over an FeO catalytic converter at liquid hydrogen temperature. This resulted in an enrichment up to 95% p-H2. A 100 ~tm nozzle and a skimmer (d=0.8 m m ) positioned 20 m m ahead of the nozzle were used. The molecular beam then passes through the electron bombarder ( 120 m m behind the skimmer), where excitation to the c aI-lu state occurs. The electron source is of the type described by Freund [ 12 ]. The cathode consists of a BaSrCO3 covered 6 m m X 3 8 m m Ni strip which is positioned alongside the molecular beam. It is indirectly heated by a tungsten wire. The anode is formed by a stack of six thin metal strips spaced 1 m m apart and arranged with their edges parallel and close ( ~ 0 . 5 ram) to the cathode. The molecular beam traverses this anode assembly parallel to the metal strips, while the electrons enter the nearly fieldfree regions between the strips. The six anode electrodes span a volume of 5 m m X 5 m m × 38 m m [ 13 ], and to confine the electron beam in the source a strong (0.25 T) magnetic field is applied parallel to the electric field between cathode and anode. In this source typically electron currents as high as 100-150 mA are obtained at 18-20 V. At this energy the excitation cross section for the electron impact excitation of the metastable c state has its maximum (see ref. [ 14] and references cited therein for a detailed discussion). The energy spread of the electron beam is in the range of 200 meV. The excitation into the c state is accompanied by a considerable too-
Volume 190, number 3,4
CHEMICALPHYSICSLETTERS
6 March 1992
H~
Fig. 2. The experimentalarrangement for the excitation of the moleculesand the saturation spectroscopy. mentum transfer to the H2 molecules. It results in a wide angular distribution of the metastables, which strongly depends on the kinetic energy of the electrons [ 15 ]. Thus, at each point of the anode volume one can imagine a divergent source of metastable molecules, leading to a diffuse, wide-aperture H2 (c) beam at the source exit. This condition calls for the application of the saturation spectroscopy technique as discussed above. The metastable molecules are excited from the c state to the n = 3 triplet states by the light of an argon-ion laser pumped ring dye laser (Coherent 699 ), which is fed into the vacuum chamber by a single mode fibre. In the main chamber a fibre outcoupling device consisting of an adjustable microscope objective ( f = 40 m m ) provides a parallel light beam with a diameter of 5 m m (fwhm). This device and all other optical components are mounted on a table which can be positioned relative to the molecular beam from outside the chamber to achieve a maxim u m LIF signal from the metastable molecules according to their angular distribution. Two 3.5 m m × 3.5 m m apertures at a distance of 50 m m from each other are used to roughly collimate the H E(C) beam. After reflection at a mirror the laser beam crosses the
H2 beam perpendicularly, 20 m m behind the second aperture, creating the fluorescence region with a volume of about 55 m m 3 at a distance of 190 m m from the exit of the electron source. The H2 n = 3 states decay radiatively to the repulsive b 3• u state, therefore the fluorescence spectrum is a broad continuum in the VUV region and the LIF photons are detected by a solar blind photomultiplier (EMI G26E314LF). It is positioned horiontally and perpendicularly to the molecular and laser beams, and its CsI photocathode ( 16 m m diameter) is at a distance of 12 m m from the center of the fluorescence region. Because the photocathode is sensitive only in the wavelength range of 110-200 nm, it is not necessary to discriminate against scattered laser light. Also the dark count rate is very low ( ~ 0.1 cps), an important advantage compared to the detection of visible laser-induced fluorescence. Between the fluorescence region and the photomultiplier a third aperture is positioned (not shown in fig. 1 ). Together with the first two apertures it shields the photomultiplier from the rather intense UV light generated in the electron source. On the opposite side of the molecular beam a small confocal high reflector (not shown in figs. 1 and 2) is mounted to enhance the LIF signal. The photomul325
Volume 190, number 3,4
CHEMICALPHYSICSLETTERS
tiplier operates in a photon counting mode, and the voltage pulses from the photomultiplier are amplified, discriminated and recorded by a Canberra 207 IA dual counter. After crossing the H2 beam the laser beam is reflected by a 90 ° prism, passes a chopper and then recrosses the H2 beam precisely antiparallel to the direction at the first crossing point. The second crossing point, where the saturation is effected, lies about 35 m m closer to the molecular beam source. It is important for the saturation technique that no laser-induced fluorescence light from the saturation region reaches the photomultiplier. This is another function of the second and third apertures. Finally a power meter serves as a laser beam stop and monitors the laser intensity, which is stable to within 10%. Around both fluorescence regions Helmholtz coils are installed (not shown in figs. 1 and 2) which compensate the horizontal component of the magnetic stray field from the electron source magnet. This is important in order to avoid a Zeeman splitting of the fine and hyperfine sublevels. Two independent pairs of coils in the two interaction regions have to be used because of the different strength of the magnetic field. The compensation of the magnetic field was performed by nulling the Zeeman splitting of a single fine structure transition at the highest spectral resolution. Metastable H2 molecules which interact with the laser light in the saturation region are effectively removed from the beam because they decay with a lifetime of 10-40 ns [ 5 ] from any n = 3 state, mostly to the repulsive b 3Eu state. Thus the fluorescence signal shows a dip, due to the reduced population of the pumped sublevel, whenever the laser frequency is scanned over a molecular transition. To produce the saturation signal, i.e. the difference between the LIF signals with and without the dip, a digital lock-in technique is used. The channel B of the dual counter is gated corresponding to the phase of the chopper, so that photons are counted only when the chopper is closed. Thus the channel B represents a standard LIF signal. In channel A all photons are counted independently of the chopper status. The saturation spectroscopy signal is then obtained as the difference 2B-A. The mirror is adjustable from outside the chamber in order to optimize the saturation signal. With the 326
6 March 1992
mirror set to only about 0.5 ° off the direction perpendicular to the molecular beam axis, the population dip is one order of magnitude smaller due to the Doppler shift, which is comparable to the Doppler width of the LIF signal (AVfwhm = 100 MHz). Thanks to the properties of the 90 ° prism, the recrossing laser beam remains exactly antiparallel to the incident beam independently of the setting of the mirror. For an absolute calibration of the light frequency a portion of the dye laser light passes through the iodine cell. The I2 LIF signal is observed by means of the photomultiplier and electrometer amplifier. Another portion of the light passes through a 1 m long confocal, passively thermally stabilized, evacuated Fabry-Perot interferometer with a free spectral range of 61.50+0.01 MHz, which provides frequency markers. Scanning the laser and storing all signals is performed by a personal computer (AT 286) via the standard IEC-Bus. It also controls the electron source current and the dye laser power. To perform measurements with highest spectral resolution, the data are taken at points separated by 2.5 MHz. The 12 bit ADC provides a maximum of about 2000 increments for the scan voltage applied to the dye laser control unit. Thus the maximum scan width is 5 GHz. With a typical dwell time of 5 s at each point, such as is necessary for a sufficient signal-to-noise, the total data accumulation time is in the range of 3 h. Because of occasional loss of the laser frequency lock point during runs as long as this, the scanning is actually done repetetively, performing, e.g., ten faster (0.5 s dwell time) scans for each measurement. Both the LIF and the saturation signal are normalized with respect to the laser power at each point. The frequency scale of each scan is linearized and calibrated by means of the Fabry-Perot interferometer markers. The individual spectra are then slightly shifted relative to each other until in each the strongest LIF features (channel B) coincide exactly. This is necessary because the starting frequencies of the individual scans can vary by about 100 MHz. The LIF peaks, although comparatively broad, can be used as standards for this adjustment because of their excellent signal-to-noise ratio. The final saturation spectrum is ultimately obtained by summing the adjusted spectra of the individual scans. In table 1 the linewidth and intensity of the strong-
Volume 190, number 3,4
CHEMICAL PHYSICS LETTERS
Table 1 Powder dependence of the saturation signal of a single fine structure transition in para-H2
Laser power (mW)
Intensity ( 103 CpS)
Linewidth
30 10 5 1 0.5 0.25
13.6 10.1 6.8 1.7 0.7 0.3
98 59 42 27 21 17
15
(MHz)
r , , , i , f , , , , , , , i , , , , , , , i f l , , , ,
~E Ck (J 10 0
"Fa ((D C
-100
0
100
Frequency [MHz] Fig. 3. The laser power dependence of the saturation signal of the transition iaH~'(v'=0, N ' = 2 , J'=3),--caH~(v"=O, N"=2, J" =3). (Upper trace: 30 mW; middle trace: 5mW; lower trace: 0.25 roW, enlarged by a factor of 10).
est single fine structure transition of the rotational line i3Fl~'(v'=0, N ' = 2 , J'=3),--c31-Iz(v"=O, N" = 2, J" = 3 ) of para-H2 are listed for various laser power levels, while in fig. 3 the corresponding saturation spectra for a laser power of 30, 5 and 0.25 mW are shown. The complete spectrum of this rotational line is shown in fig. 5 and will be discussed below. Note that at 30 mW the line profile in fig. 3 is clearly non-Lorentzian (a fit in the wings of the profile gives a peak height ~ 1.6 times greater than measured). For a theoretical treatment of this line profile deformation, see ref. [ 16 ]. It is seen in table 1 that a decrease of the laser power by a factor of 100 results in a decrease of the saturation signal and of the power broadened linewidth by factors of 40 and 6, respectively. In the limit of zero laser power the
6 March 1992
linewidth is in the range of the natural linewidth, which is about 12 MHz as calculated from the lifetimes given in ref. [ 5 ]. Thus the ultimate limit of the linewidth can actually be reached in this experiment. On the other hand, if the highest possible resolution is not required, the optimum compromise between intensity and linewidth can be found very conveniently by simply adjusting the laser power. In the collimation technique a lower limit of the line width is fixed by the the slit width used. As an example of a complete rovibrational line, fig. 4 shows the spectrum of the i3I-I~-(v'=0, N ' = l ) ~ c 3 I T ~ ( v " = O , N " = I ) line of ortho-H2 measured with a laser power of 3 mW. The Dopplerbroadened LIF spectrum is shown in the upper trace, while the lower trace gives the corresponding saturation signal. The overall rotational line profile shows both fine and hyperfine structure. The clustering of the peaks into two groups is due to the large fine structure splitting of the c state, which is precisely known from ref. [2] and is about 5 GHz. The hyperfine structure of this lower level and the fine and hyperfine structure of the upper level are smaller by one order of magnitude, in accordance with the narrower spacings between the line components within each group. Experimentally the saturation measurement is performed in two separate sections. There are two reasons for doing this. On the one hand it saves time to skip the frequency interval between the two groups of lines, where no transitions can occur. On the other hand the maximum scan range for measurements with highest resolution is about 5 G H z as discussed above. Therefore the lower spectrum of fig. 4 shows two unconnected measurements. The upper trace, on the other hand, was measured in a separate continuous scan, using somewhat larger frequency steps. It illustrates the very good signal-to-noise ratio of the LIF signal achieved even in a single scan (i.e. without summation of repetetive scan data). The different line widths of the peaks in the saturation spectrum are mainly due to the unresolved hyperfine components hidden in these peaks. A comparison of the relative intensities in the two spectra shows that in general transitions with a large transition probability are relatively more saturated. The peak heights are, however, not strictly proportional to each other. For example, the peaks at 2.8 and 4.7 G H z are rel327
Volume 190, n u m b e r 3,4
4o
CHEMICAL PHYSICS LETTERS
, , , i , , , , , , , , , i , , , , , , , , , i , , , , , , , , , i , , , i , , ,
6 March 1992
,/
0~ 30
2.0
EL
Q_ 0
F'Q
1.5
0
5
20 LIF
0 cO7
~
i
,
i
.
~.o
i
-if? ~ 0
10
°_
0.5
©
L D 0
3.9
-4
-2
0
2
Frequency Fig. 4. The LIF a n d saturation signal of the transition frequency scale refers to the n o m i n a l line center.
6
0.0
4.1
8
[CHz]
i 31-I+ (v' = 0, N'
atively much stronger in the LIF spectrum than in the saturation signal. This might be explained by the assumption that such peaks comprise several hyperfine components, each having a small transition probability and thus resulting in a very small saturation intensity. A confirmation will have to await a complete assignment of the hyperfine transitions involved. The inset in fig. 4 shows the saturation signal of the peak at 4.0 GHz with even higher resolution, which was measured with a laser power of only 1 mW. It demonstrates again clearly the resolving power of the saturation technique. In fig. 5 we present the saturation signal of the rotational line i 3I-I; (/3' = 0, N ' = 2 ) ,-- c 31I u ( V" = 0 , N " = 2) in para-H2 measured with a laser power of l0 mW, which shows the completely resolved fine structure. This is a line which had also been measured by the depletion technique [ 3,4 ]. However, a comparison with fig. A.IV-2 ofref. [4] demonstrates the greatly superior signal-to-noise ratio of the present work. The assignment of each component, following ref. [ 4 ], is also given in fig. 5, and the theoretical intensity distribution is shown in the form of a bar diagram. The intensities of the different fine structure transitions have been calculated from the matrix element of the dipole operator d in the form 328
4
4.O
= 1 ) , - c ~FI [ ( v" = 0, N" = 1 ) at 17o= 17212.007 ( 1 )
cm -L
The
(A',N',S,J'[dlA",N",S,J") = ( -- 1 ) N ' + I + S + J " [
×
S
J'
( 2 J ' + 1 ) (2J" + 1 ) ],/2
(A',N'IIdlIA",N").
where {...} is the Wigner 6j-symbol and ( A ' , N' IIdllA", N" ) the reduced matrix element, which is constant for all fine structure components of a rotational line. The intensity distribution shows clearly that the transitions with 6,/= AN are the most intense, in accordance with a well-known rule. The spectrum shows also several additional peaks, which are crossover resonances and are characteristic for any saturation spectroscopy method. They occur always midway between two transitions having a common lower level, provided they fall within the Doppler width of the fluorescence signal. Crossover resonances are helpful in assigning transitions, but if the transitions are too close, they can degrade the resolution considerably. In the bar diagram they are shown with a negative intensity, while the height of these bars is adjusted according to the experimental spectrum. These two examples of highly resolved rotational lines in para-H2 (exhibiting only fine structure split-
Volume 190, number 3,4
CHEMICAL PHYSICS LETTERS
. . . .
, , 1 1
r
. . . .
6 March 1992
,
°(..)- 1 5 0
~ 0")
~ (D
10 5
E
0 J' j"
2 1 2 2
3 2
2 1 1 1
2 3
3 3
FS-components [7
Crossover i
i
I
-4.(?
i
i
L
, , , 1 1 , ,
-5.5
hJ . . . .
1.0 Frequency
L,,
1.5 [OHz]
L~I
, , I , ,
2.0
2.5
Fig. 5. The saturation signal of the transition i 31-1~ (v' = 0, N' = 2, J' ),- c 3If~ (v" = 0, N" = 2, J" ) at ~7o= 17248.515 (3) cm - ~. The frequency scale referes to the nominal line center. ting) as well as in ortho-H2 (with both fine a n d hyperfine structure) show that the technique described here is a powerful source o f detailed i n f o r m a t i o n on the electronic structure o f the strongly m i x e d states o f the n = 3 triplet complex. W i t h the i m p r o v e d resolution a n d sensitively o f the present e x p e r i m e n t it has been possible to exceed significantly the range o f the m e a s u r e m e n t s o f Lichten et al. [ 3,4 ], who obtained the spectra from c 3 I - I u ( v = 0 a n d 1, N = 2 ) only. The results already o b t a i n e d include the rotational level N = 4 o f v = 0 . This is i m p o r t a n t for the d e t e r m i n a t i o n o f a complete set o f fine structure coupling constants. The limited n u m b e r o f fine structure splittings m e a s u r e d in ref. [3,4] is insufficient for an unambiguous d e t e r m i n a t i o n o f these constants. The experiments are at present being extended to the rotational levels N = 1 a n d 3, v = 0, o f ortho-H2 and to N = 1-4 o f the A v = 0 bands o f vibrational levels with v> 0. A p a r t from the isolated m e a s u r e m e n t o f only two rotational lines in ref. [ 7 ], the present work gives for the first t i m e the hyperfine splittings o f the transitions to the H2 n = 3 triplet states. The resolution is about one o r d e r o f magnitude better than o b t a i n e d in ref. [7 ]. The quantitative analysis o f the hyperfine structure o f the ortho-H2 spectra such as shown in fig. 4 will be based
on the known hyperfine splitting o f the lower (c) state from ref. [ 2 ]. It will then p r o v i d e detailed inf o r m a t i o n about the unknown hyperfine interaction within the n = 3 triplet states. Support o f this work by the Sonderforschungsbereich 93 " P h o t o c h e m i s t r y with Lasers" is greatfully acknowledged. We also thank Dr. A. S h a r m a for the stimulating discussions.
References [ 1] W. Lichten, Phys. Rev. 120 (1960) 848. [ 2 ] W. Lichten and T. Wik, J. Chem. Phys. 69 ( 1978 ) 5428. [3] W. Lichten, T. Wik and T.A. Miller, J. Chem. Phys. 71 (1979) 2441. [4] T. Wik, Dissertation, Yale University, 1977. [ 5 ] E.E. Eyler and F.M. Pipkin, Phys. Rev. Letters 47 ( 1981 ) 1270. [ 6 ] E.E. Eyler and F.M. Pipkin, J. Chem. Phys. 77 (1982) 5315. [7] N. Bjerre and H. Helm, Chem. Phys. Letters 134 (1987) 361. [8] W. Koot, W.J. Van der Zande, P.H.P. Post and J. Los, J. Chem. Phys. 90 (1989) 4826. [ 9 ] H. Geisen, D. Neuschiifer and Ch. Ottinger, Z. Physik D 17 (1990) 137. [ 10] W. Demtr'6der, Laser spectroscopy (Springer, Berlin, 1982). 329
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CHEMICAL PHYSICS LETTERS
[ 11 ] H. Geisen, T. Kriimpelmann, D. Neusch~ifer and Ch. Ottinger, Z. Naturforsch. 42a (1987) 519. [ 12] R. Freund, Rev. Sci. Instr. 41 (1970) 1213. [ 13 ] W. B6hle, Diplom. Thesis, G/Sttingen, 1986. [14] Ch. Ottinger and T. Rox, Phys. Letters, submitted for publication.
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[ 15 ] S. Ohshima, T. Kondow, T. Fukuyama and H. Kuchitsu, Chem. Phys. Letters 169 (1990) 331. [ 16] N. Billy, B. Girard, G. Gouedard and J. Vigue, Mol. Phys. 61 (1987) 65.
CHEMICALPHYSICSLETTERS
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High-resolution saturation spectroscopy of the H2 n = 3 triplet states L. Jozefowski, Ch. Ottinger a n d T. R o x Max-Planck-lnstitut far StrOmungsforschung, Bunsenstrasse 10, W-3400 G6ttingen, Germany
Received 25 October 1991
For measurementsof the fine and hyperfinestructure of the H2 n = 3 complexof triplet states, laser saturation spectroscopyhas been applied to a beam of metastable H 2 molecules. A spectral resolution of better than 20 MHz (fwhm) was achieved. The experimental arrangement is described in detail, and preliminary results are given.
The electronic structure of highly excited Rydberg states of H2 m'olecules is of fundamental interest. It can be elucidated by means of high-resolution laser spectroscopy. In particular, the four strongly mixed triplet states with n = 3 , (3s)g3Z~, (3d)h 3Z~-, (3d)i 3FI~ and (3d)j 3A~ which lie at Te~, 13.8 eV, are accessible from the metastable c 31-I~- state by absorption of visible laser light. This offers the possibility of determining accurately the absolute excitation energies as well as the fine and hyperfine structure of specific rovibrational levels. The metastable character of the c state itself was discovered by Lichten [ 1 ] in 1960 who also measured its fine and hyperfine structure by means of the molecular beam magnetic resonance technique [ 2 ]. Since then, there have been various laser spectroscopic investigations on beams of H 2 (C). The fine structure of the higherlying n = 3 triplet states was measured for some levels (v' = 0 and 1, N' = 2) of para-H2 by Lichten's group [3,4]. They used a sub-Doppler resolution technique based on the partial depletion of the H2 (c) beam by laser optical pumping. Eyler and Pipkin determined the radiative lifetime of the n = 3 and 4 states by means of laser-induced fluorescence (LIF) [5,6]. In both studies the same type of molecular beam apparatus was used, where HE molecules are excited by electron impact to the metastable c state. Bjerre and Helm [ 7 ] partially resolved the fine and hyperfine structure of two rotational lines of the HE(g~c, v " = 2 ) transition using the fast neutral
beam photofragment spectroscopy. Here H 2 (C) molecules are generated by neutralization of H~-. In the same manner Koot et al. [8 ] determined experimental wavefunctions of the excited states and analyzed the dissociation dynamics. High-resolution laser spectroscopy on molecular beams is normally done using narrow slits which collimate the molecular beam in the direction of the laser light, so that the Doppler broadening is minimized. For example, Geisen et al. [ 9 ] obtained a resolution of 8 MHz, mainly determined by the residual Doppler width, in measurements of the hyperfine structure of the B3FIg and A 3E ,+ states of molecular nitrogen. Lichten et al. [ 3 ], using the same method in their spectroscopy of the n = 3 triplet states of H2, reached a minimum Doppler width of about 16 MHz, additional to the natural linewidth and the power broadening. Another well-known method of achieving very high, Doppler-free resolution in laser spectroscopy is the saturation technique, with its variants of absorption, intermodulated fluorescence or polarization detection [ 10 ]. This technique is most often used in cell experiments, but it also offers the high resolution power in combination with molecular beams, as has been reported earlier [ 11 ]. With this method there is no need for any beam collimation, because the saturation signal automatically selects all those molecules whose velocity is precisely perpendicular to the laser beam. This is true even in the case of a wide molecular beam issuing from a large
0009-2614/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
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CHEMICALPHYSICSLETTERS
volume. The entire cross section of such a beam will contribute to the saturation signal. It is in this situation that the saturation technique is greatly superior to the collimation method. The latter would require two slits, both much narrower than the extended beam source, and would thus utilize only a small fraction of the total beam intensity. The present experiment is of the extended, diffuse source variety, therefore the saturation technique is very advantageous in providing a Doppler-free signal with a much larger intensity. An overview of the whole experimental arrangement of the present experiment is given in fig. l, while fig. 2 shows a perspective drawing of the parts essential to the saturation technique. The apparatus consists of two vacuum chambers made of stainless nonmagnetic steel. The nozzle chamber is pumped
Dye Laser
oHt!,
NoS c _l A1
A2
'
Fig. 1. Schematic overview of the experiment. A: anode; A 1, A2: apertures; A/D: amplifier and discriminator; C: cathode; Ch: chopper; FO: fiber optics; FPI: Fabry-Perot interferometer; G: gate; M: mirror; N,S: magnet; No: nozzle; PM: photomultiplier; PoM: powermeter; Sk: skimmer.
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by an oil diffusion pump with a water cooled baffle, having an overall pumping speed of 3000 ~/s. The H2 background pressure in the chambers is about 5 × 10 -4 mbar. The main chamber is pumped by an oil diffusion pump with a freon cooled baffle, giving a pumping speed of 1500 I~/s. The pressure here is about 5 × 10 -6 mbar. Both pumps are backed by a 500 m3/h Roots and a 180 ma/h rotary pump. For ortho-H2 one can observe both fine and hyperfine structure, while for para-H2, which has zero total nuclear spin, only fine structure occurs. The possibility of preparing alternatively natural and para-H2 enriched gas proved to be a valuable aid in the analysis of the spectra. The natural hydrogen (99.99% purity) was used as supplied in a 200 bar cylinder, with a regulator valve providing the inlet pressure of 2.5 bar. The para-H2 enriched hydrogen was generated from liquid hydrogen by passing the evaporating gas over an FeO catalytic converter at liquid hydrogen temperature. This resulted in an enrichment up to 95% p-H2. A 100 ~tm nozzle and a skimmer (d=0.8 m m ) positioned 20 m m ahead of the nozzle were used. The molecular beam then passes through the electron bombarder ( 120 m m behind the skimmer), where excitation to the c aI-lu state occurs. The electron source is of the type described by Freund [ 12 ]. The cathode consists of a BaSrCO3 covered 6 m m X 3 8 m m Ni strip which is positioned alongside the molecular beam. It is indirectly heated by a tungsten wire. The anode is formed by a stack of six thin metal strips spaced 1 m m apart and arranged with their edges parallel and close ( ~ 0 . 5 ram) to the cathode. The molecular beam traverses this anode assembly parallel to the metal strips, while the electrons enter the nearly fieldfree regions between the strips. The six anode electrodes span a volume of 5 m m X 5 m m × 38 m m [ 13 ], and to confine the electron beam in the source a strong (0.25 T) magnetic field is applied parallel to the electric field between cathode and anode. In this source typically electron currents as high as 100-150 mA are obtained at 18-20 V. At this energy the excitation cross section for the electron impact excitation of the metastable c state has its maximum (see ref. [ 14] and references cited therein for a detailed discussion). The energy spread of the electron beam is in the range of 200 meV. The excitation into the c state is accompanied by a considerable too-
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6 March 1992
H~
Fig. 2. The experimentalarrangement for the excitation of the moleculesand the saturation spectroscopy. mentum transfer to the H2 molecules. It results in a wide angular distribution of the metastables, which strongly depends on the kinetic energy of the electrons [ 15 ]. Thus, at each point of the anode volume one can imagine a divergent source of metastable molecules, leading to a diffuse, wide-aperture H2 (c) beam at the source exit. This condition calls for the application of the saturation spectroscopy technique as discussed above. The metastable molecules are excited from the c state to the n = 3 triplet states by the light of an argon-ion laser pumped ring dye laser (Coherent 699 ), which is fed into the vacuum chamber by a single mode fibre. In the main chamber a fibre outcoupling device consisting of an adjustable microscope objective ( f = 40 m m ) provides a parallel light beam with a diameter of 5 m m (fwhm). This device and all other optical components are mounted on a table which can be positioned relative to the molecular beam from outside the chamber to achieve a maxim u m LIF signal from the metastable molecules according to their angular distribution. Two 3.5 m m × 3.5 m m apertures at a distance of 50 m m from each other are used to roughly collimate the H E(C) beam. After reflection at a mirror the laser beam crosses the
H2 beam perpendicularly, 20 m m behind the second aperture, creating the fluorescence region with a volume of about 55 m m 3 at a distance of 190 m m from the exit of the electron source. The H2 n = 3 states decay radiatively to the repulsive b 3• u state, therefore the fluorescence spectrum is a broad continuum in the VUV region and the LIF photons are detected by a solar blind photomultiplier (EMI G26E314LF). It is positioned horiontally and perpendicularly to the molecular and laser beams, and its CsI photocathode ( 16 m m diameter) is at a distance of 12 m m from the center of the fluorescence region. Because the photocathode is sensitive only in the wavelength range of 110-200 nm, it is not necessary to discriminate against scattered laser light. Also the dark count rate is very low ( ~ 0.1 cps), an important advantage compared to the detection of visible laser-induced fluorescence. Between the fluorescence region and the photomultiplier a third aperture is positioned (not shown in fig. 1 ). Together with the first two apertures it shields the photomultiplier from the rather intense UV light generated in the electron source. On the opposite side of the molecular beam a small confocal high reflector (not shown in figs. 1 and 2) is mounted to enhance the LIF signal. The photomul325
Volume 190, number 3,4
CHEMICALPHYSICSLETTERS
tiplier operates in a photon counting mode, and the voltage pulses from the photomultiplier are amplified, discriminated and recorded by a Canberra 207 IA dual counter. After crossing the H2 beam the laser beam is reflected by a 90 ° prism, passes a chopper and then recrosses the H2 beam precisely antiparallel to the direction at the first crossing point. The second crossing point, where the saturation is effected, lies about 35 m m closer to the molecular beam source. It is important for the saturation technique that no laser-induced fluorescence light from the saturation region reaches the photomultiplier. This is another function of the second and third apertures. Finally a power meter serves as a laser beam stop and monitors the laser intensity, which is stable to within 10%. Around both fluorescence regions Helmholtz coils are installed (not shown in figs. 1 and 2) which compensate the horizontal component of the magnetic stray field from the electron source magnet. This is important in order to avoid a Zeeman splitting of the fine and hyperfine sublevels. Two independent pairs of coils in the two interaction regions have to be used because of the different strength of the magnetic field. The compensation of the magnetic field was performed by nulling the Zeeman splitting of a single fine structure transition at the highest spectral resolution. Metastable H2 molecules which interact with the laser light in the saturation region are effectively removed from the beam because they decay with a lifetime of 10-40 ns [ 5 ] from any n = 3 state, mostly to the repulsive b 3Eu state. Thus the fluorescence signal shows a dip, due to the reduced population of the pumped sublevel, whenever the laser frequency is scanned over a molecular transition. To produce the saturation signal, i.e. the difference between the LIF signals with and without the dip, a digital lock-in technique is used. The channel B of the dual counter is gated corresponding to the phase of the chopper, so that photons are counted only when the chopper is closed. Thus the channel B represents a standard LIF signal. In channel A all photons are counted independently of the chopper status. The saturation spectroscopy signal is then obtained as the difference 2B-A. The mirror is adjustable from outside the chamber in order to optimize the saturation signal. With the 326
6 March 1992
mirror set to only about 0.5 ° off the direction perpendicular to the molecular beam axis, the population dip is one order of magnitude smaller due to the Doppler shift, which is comparable to the Doppler width of the LIF signal (AVfwhm = 100 MHz). Thanks to the properties of the 90 ° prism, the recrossing laser beam remains exactly antiparallel to the incident beam independently of the setting of the mirror. For an absolute calibration of the light frequency a portion of the dye laser light passes through the iodine cell. The I2 LIF signal is observed by means of the photomultiplier and electrometer amplifier. Another portion of the light passes through a 1 m long confocal, passively thermally stabilized, evacuated Fabry-Perot interferometer with a free spectral range of 61.50+0.01 MHz, which provides frequency markers. Scanning the laser and storing all signals is performed by a personal computer (AT 286) via the standard IEC-Bus. It also controls the electron source current and the dye laser power. To perform measurements with highest spectral resolution, the data are taken at points separated by 2.5 MHz. The 12 bit ADC provides a maximum of about 2000 increments for the scan voltage applied to the dye laser control unit. Thus the maximum scan width is 5 GHz. With a typical dwell time of 5 s at each point, such as is necessary for a sufficient signal-to-noise, the total data accumulation time is in the range of 3 h. Because of occasional loss of the laser frequency lock point during runs as long as this, the scanning is actually done repetetively, performing, e.g., ten faster (0.5 s dwell time) scans for each measurement. Both the LIF and the saturation signal are normalized with respect to the laser power at each point. The frequency scale of each scan is linearized and calibrated by means of the Fabry-Perot interferometer markers. The individual spectra are then slightly shifted relative to each other until in each the strongest LIF features (channel B) coincide exactly. This is necessary because the starting frequencies of the individual scans can vary by about 100 MHz. The LIF peaks, although comparatively broad, can be used as standards for this adjustment because of their excellent signal-to-noise ratio. The final saturation spectrum is ultimately obtained by summing the adjusted spectra of the individual scans. In table 1 the linewidth and intensity of the strong-
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CHEMICAL PHYSICS LETTERS
Table 1 Powder dependence of the saturation signal of a single fine structure transition in para-H2
Laser power (mW)
Intensity ( 103 CpS)
Linewidth
30 10 5 1 0.5 0.25
13.6 10.1 6.8 1.7 0.7 0.3
98 59 42 27 21 17
15
(MHz)
r , , , i , f , , , , , , , i , , , , , , , i f l , , , ,
~E Ck (J 10 0
"Fa ((D C
-100
0
100
Frequency [MHz] Fig. 3. The laser power dependence of the saturation signal of the transition iaH~'(v'=0, N ' = 2 , J'=3),--caH~(v"=O, N"=2, J" =3). (Upper trace: 30 mW; middle trace: 5mW; lower trace: 0.25 roW, enlarged by a factor of 10).
est single fine structure transition of the rotational line i3Fl~'(v'=0, N ' = 2 , J'=3),--c31-Iz(v"=O, N" = 2, J" = 3 ) of para-H2 are listed for various laser power levels, while in fig. 3 the corresponding saturation spectra for a laser power of 30, 5 and 0.25 mW are shown. The complete spectrum of this rotational line is shown in fig. 5 and will be discussed below. Note that at 30 mW the line profile in fig. 3 is clearly non-Lorentzian (a fit in the wings of the profile gives a peak height ~ 1.6 times greater than measured). For a theoretical treatment of this line profile deformation, see ref. [ 16 ]. It is seen in table 1 that a decrease of the laser power by a factor of 100 results in a decrease of the saturation signal and of the power broadened linewidth by factors of 40 and 6, respectively. In the limit of zero laser power the
6 March 1992
linewidth is in the range of the natural linewidth, which is about 12 MHz as calculated from the lifetimes given in ref. [ 5 ]. Thus the ultimate limit of the linewidth can actually be reached in this experiment. On the other hand, if the highest possible resolution is not required, the optimum compromise between intensity and linewidth can be found very conveniently by simply adjusting the laser power. In the collimation technique a lower limit of the line width is fixed by the the slit width used. As an example of a complete rovibrational line, fig. 4 shows the spectrum of the i3I-I~-(v'=0, N ' = l ) ~ c 3 I T ~ ( v " = O , N " = I ) line of ortho-H2 measured with a laser power of 3 mW. The Dopplerbroadened LIF spectrum is shown in the upper trace, while the lower trace gives the corresponding saturation signal. The overall rotational line profile shows both fine and hyperfine structure. The clustering of the peaks into two groups is due to the large fine structure splitting of the c state, which is precisely known from ref. [2] and is about 5 GHz. The hyperfine structure of this lower level and the fine and hyperfine structure of the upper level are smaller by one order of magnitude, in accordance with the narrower spacings between the line components within each group. Experimentally the saturation measurement is performed in two separate sections. There are two reasons for doing this. On the one hand it saves time to skip the frequency interval between the two groups of lines, where no transitions can occur. On the other hand the maximum scan range for measurements with highest resolution is about 5 G H z as discussed above. Therefore the lower spectrum of fig. 4 shows two unconnected measurements. The upper trace, on the other hand, was measured in a separate continuous scan, using somewhat larger frequency steps. It illustrates the very good signal-to-noise ratio of the LIF signal achieved even in a single scan (i.e. without summation of repetetive scan data). The different line widths of the peaks in the saturation spectrum are mainly due to the unresolved hyperfine components hidden in these peaks. A comparison of the relative intensities in the two spectra shows that in general transitions with a large transition probability are relatively more saturated. The peak heights are, however, not strictly proportional to each other. For example, the peaks at 2.8 and 4.7 G H z are rel327
Volume 190, n u m b e r 3,4
4o
CHEMICAL PHYSICS LETTERS
, , , i , , , , , , , , , i , , , , , , , , , i , , , , , , , , , i , , , i , , ,
6 March 1992
,/
0~ 30
2.0
EL
Q_ 0
F'Q
1.5
0
5
20 LIF
0 cO7
~
i
,
i
.
~.o
i
-if? ~ 0
10
°_
0.5
©
L D 0
3.9
-4
-2
0
2
Frequency Fig. 4. The LIF a n d saturation signal of the transition frequency scale refers to the n o m i n a l line center.
6
0.0
4.1
8
[CHz]
i 31-I+ (v' = 0, N'
atively much stronger in the LIF spectrum than in the saturation signal. This might be explained by the assumption that such peaks comprise several hyperfine components, each having a small transition probability and thus resulting in a very small saturation intensity. A confirmation will have to await a complete assignment of the hyperfine transitions involved. The inset in fig. 4 shows the saturation signal of the peak at 4.0 GHz with even higher resolution, which was measured with a laser power of only 1 mW. It demonstrates again clearly the resolving power of the saturation technique. In fig. 5 we present the saturation signal of the rotational line i 3I-I; (/3' = 0, N ' = 2 ) ,-- c 31I u ( V" = 0 , N " = 2) in para-H2 measured with a laser power of l0 mW, which shows the completely resolved fine structure. This is a line which had also been measured by the depletion technique [ 3,4 ]. However, a comparison with fig. A.IV-2 ofref. [4] demonstrates the greatly superior signal-to-noise ratio of the present work. The assignment of each component, following ref. [ 4 ], is also given in fig. 5, and the theoretical intensity distribution is shown in the form of a bar diagram. The intensities of the different fine structure transitions have been calculated from the matrix element of the dipole operator d in the form 328
4
4.O
= 1 ) , - c ~FI [ ( v" = 0, N" = 1 ) at 17o= 17212.007 ( 1 )
cm -L
The
(A',N',S,J'[dlA",N",S,J") = ( -- 1 ) N ' + I + S + J " [
×
S
J'
( 2 J ' + 1 ) (2J" + 1 ) ],/2
(A',N'IIdlIA",N").
where {...} is the Wigner 6j-symbol and ( A ' , N' IIdllA", N" ) the reduced matrix element, which is constant for all fine structure components of a rotational line. The intensity distribution shows clearly that the transitions with 6,/= AN are the most intense, in accordance with a well-known rule. The spectrum shows also several additional peaks, which are crossover resonances and are characteristic for any saturation spectroscopy method. They occur always midway between two transitions having a common lower level, provided they fall within the Doppler width of the fluorescence signal. Crossover resonances are helpful in assigning transitions, but if the transitions are too close, they can degrade the resolution considerably. In the bar diagram they are shown with a negative intensity, while the height of these bars is adjusted according to the experimental spectrum. These two examples of highly resolved rotational lines in para-H2 (exhibiting only fine structure split-
Volume 190, number 3,4
CHEMICAL PHYSICS LETTERS
. . . .
, , 1 1
r
. . . .
6 March 1992
,
°(..)- 1 5 0
~ 0")
~ (D
10 5
E
0 J' j"
2 1 2 2
3 2
2 1 1 1
2 3
3 3
FS-components [7
Crossover i
i
I
-4.(?
i
i
L
, , , 1 1 , ,
-5.5
hJ . . . .
1.0 Frequency
L,,
1.5 [OHz]
L~I
, , I , ,
2.0
2.5
Fig. 5. The saturation signal of the transition i 31-1~ (v' = 0, N' = 2, J' ),- c 3If~ (v" = 0, N" = 2, J" ) at ~7o= 17248.515 (3) cm - ~. The frequency scale referes to the nominal line center. ting) as well as in ortho-H2 (with both fine a n d hyperfine structure) show that the technique described here is a powerful source o f detailed i n f o r m a t i o n on the electronic structure o f the strongly m i x e d states o f the n = 3 triplet complex. W i t h the i m p r o v e d resolution a n d sensitively o f the present e x p e r i m e n t it has been possible to exceed significantly the range o f the m e a s u r e m e n t s o f Lichten et al. [ 3,4 ], who obtained the spectra from c 3 I - I u ( v = 0 a n d 1, N = 2 ) only. The results already o b t a i n e d include the rotational level N = 4 o f v = 0 . This is i m p o r t a n t for the d e t e r m i n a t i o n o f a complete set o f fine structure coupling constants. The limited n u m b e r o f fine structure splittings m e a s u r e d in ref. [3,4] is insufficient for an unambiguous d e t e r m i n a t i o n o f these constants. The experiments are at present being extended to the rotational levels N = 1 a n d 3, v = 0, o f ortho-H2 and to N = 1-4 o f the A v = 0 bands o f vibrational levels with v> 0. A p a r t from the isolated m e a s u r e m e n t o f only two rotational lines in ref. [ 7 ], the present work gives for the first t i m e the hyperfine splittings o f the transitions to the H2 n = 3 triplet states. The resolution is about one o r d e r o f magnitude better than o b t a i n e d in ref. [7 ]. The quantitative analysis o f the hyperfine structure o f the ortho-H2 spectra such as shown in fig. 4 will be based
on the known hyperfine splitting o f the lower (c) state from ref. [ 2 ]. It will then p r o v i d e detailed inf o r m a t i o n about the unknown hyperfine interaction within the n = 3 triplet states. Support o f this work by the Sonderforschungsbereich 93 " P h o t o c h e m i s t r y with Lasers" is greatfully acknowledged. We also thank Dr. A. S h a r m a for the stimulating discussions.
References [ 1] W. Lichten, Phys. Rev. 120 (1960) 848. [ 2 ] W. Lichten and T. Wik, J. Chem. Phys. 69 ( 1978 ) 5428. [3] W. Lichten, T. Wik and T.A. Miller, J. Chem. Phys. 71 (1979) 2441. [4] T. Wik, Dissertation, Yale University, 1977. [ 5 ] E.E. Eyler and F.M. Pipkin, Phys. Rev. Letters 47 ( 1981 ) 1270. [ 6 ] E.E. Eyler and F.M. Pipkin, J. Chem. Phys. 77 (1982) 5315. [7] N. Bjerre and H. Helm, Chem. Phys. Letters 134 (1987) 361. [8] W. Koot, W.J. Van der Zande, P.H.P. Post and J. Los, J. Chem. Phys. 90 (1989) 4826. [ 9 ] H. Geisen, D. Neuschiifer and Ch. Ottinger, Z. Physik D 17 (1990) 137. [ 10] W. Demtr'6der, Laser spectroscopy (Springer, Berlin, 1982). 329
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[ 11 ] H. Geisen, T. Kriimpelmann, D. Neusch~ifer and Ch. Ottinger, Z. Naturforsch. 42a (1987) 519. [ 12] R. Freund, Rev. Sci. Instr. 41 (1970) 1213. [ 13 ] W. B6hle, Diplom. Thesis, G/Sttingen, 1986. [14] Ch. Ottinger and T. Rox, Phys. Letters, submitted for publication.
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[ 15 ] S. Ohshima, T. Kondow, T. Fukuyama and H. Kuchitsu, Chem. Phys. Letters 169 (1990) 331. [ 16] N. Billy, B. Girard, G. Gouedard and J. Vigue, Mol. Phys. 61 (1987) 65.