IV.
ATOMIC RESONANCE AND SCATTERING
Academic and Research Staff
Prof. Daniel Kleppner
Prof. David E. Pritchard
Prof. Dieter Zimmermann
Dr. Theodore W. Ducas
Dr. Richard R. Freeman
Dr. Rudolph G. Suchannek
Graduate Students
Riad N. Ahmad
Jerome Apt III
Timothy A. Brunner
William E. Cooke
A.
Martin C. Kaplan
Walter P. Lapatovich
Michael G. Littman
William D. Phillips
Naser S. Saleh
John A. Serri
Myron L. Zimmerman
OPTICAL FREQUENCY STANDARD
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Joint Services Electronics Program (Contract DAAB07-75-C-1346)
Riad N.
Ahmad, Walter P.
Lapatovich,
David E.
T
Pritchard
We have taken test spectra of Na
marily of Ar or Ne.
2 in a supersonic molecular beam consisting priUse of a supersonic molecular beam has two advantages: the Dop-
pler width of the transition is reduced because the beam motion is perpendicular to the
laser beam, and the gas expansion cools the molecules into the lowest vibrational and
rotational states. A sample spectrum is shown in Fig. IV-1.
IA
Fig. IV-1.
Soectrum of seeded Na. molecular beam near 6020 A. N
Z
0
'D
(positive) marker pulses are separated 0. 019 A (0. 159 A).
The test spectra show a 0. 002 A linewidth.
approximately +0. 001 A (100 MHz).
PR No.
118
etiv
--
From day to day, the line position varies
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B.
ATOMIC RESONANCE AND SCATTERING)
VELOCITY DEPENDENCE OF ENERGY TRANSFER CROSS SECTIONS
Joint Services Electronics Program (Contract DAABO7-75-C-1346)
Jerome Apt, David E. Pritchard
We have measured the velocity dependence of fine-structure changing cross sections
between Na and the target gases Ne, Ar, Kr, Xe, N 2 , and CO 2 . These measurements
1
were made with the use of our technique of velocity selection based on the Doppler shift
which permits the measurement of the velocity dependence of collision cross sections
Na +Xe
A
AA
-7,
4At
-
x
I
I
31
I
I
I
i
L
t
0.5
0_
F
0.2 5o
31
.8
1.2
i.0
VELOCITY (105
Fig. IV-2.
I
I
c
I
I
14
1.6
m/sec)
Velocity dependence of Na-Xe fine-structure changing cross
3
1
section for j =~-
= -.
Lower section shows rms velocity
resolution achieved by our velocity-selection Doppler-shift
(VSDS) technique.
in a gas cell, in contrast to the usual and more cumbersome crossed molecular beam
machines. Typical results are shown in Fig. IV-2 for the process
Na(3P 1 /2) + Xe - Na(3P
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+ Xe - AE
where AE is the amount of translational energy that goes into exciting the fine structure
PR No.
118
(IV.
ATOMIC RESONANCE AND SCATTERING)
of Na.
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Measurements of fine-structure changing collisions such as this one are important
both for comparison with recent theory 2 and because this process is an important collision process in fine-structure lasers such as the high-power atomic iodine laser.
References
1.
W. D.
2.
J.
C.
ELECTRIC FIELD IONIZATION RATES OF SELECTED
STARK STATES IN SODIUM
Phillips and D. E.
Pascale and R. E.
Pritchard, Phys. Rev. Letters 33,
Olson,
J.
Chem.
Phys. 64,
1254-1257 (1974).
3538 (1976).
Joint Services Electronics Program (Contract DAAB07-75-C-1346)
Michael G.
Littman,
Myron L.
Zimmerman,
Daniel Kleppner
We have undertaken to study electric field ionization by tunneling in Rydberg states
n = 12-15.
We have studied the rm! = 2 states, which are well described
of sodium,
by hydrogenic theory.
The theory of ionization by tunneling has been developed by sevIn particular, Bailey, Hiskes, and Riviere have calculated rates for
eral workers.
states up to n = 25.
ment with theory.
Early spectroscopic observations for low levels (n<
6
) are in agree-
Field ionization studies for statistical population distributions of
Stark states with larger values of n have been carried out,
5
' 6
but to our knowledge
there have been no direct measurements of spontaneous ionization rates from selected
Stark sublevels.
beam.
Using pulsed excitation, we have observed ionization of atoms in a
The apparatus has been described elsewhere.
7
, 8 Ionization rates were mea-
sured by direct observation of ion current vs time and in some instances by observation
of level broadening.
The lowest level in each manifold ionizes at a rate that is in general agreement
with the results of Bailey et al. 4
ment.
For many levels, however, we find striking disagreeTheir theory predicts that ionization rates increase with field in a rapid mono-
tonic fashion and the redder states of a given manifold (i. e.,
ionization limit) ionize first as the field is increased.
those farthest from the
We find that the bluer compo-
nents ionize first, the rates for these states are not monotonic,
and the onset of ioniza-
tion occurs at fields that in some cases are a factor of 2-3 smaller than predicted.
The anomalous
lying terms.
ionization phenomena appear to be due to interactions with higher
The essential mechanism is as follows:
numerous level anticrossings
can occur before the level of interest ionizes according to the theory of Bailey et al.
If
the anticrossing occurs with a level from a higher term with a large ionization rate, then
the level of interest will also ionize because of level mixing.
PR No.
118
Data
exhibiting these
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ATOMIC RESONANCE AND SCATTERING)
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i
t
io
I
I
.
I
F
pheomwin
I
o
LL13-
12-
I
I
I
I
680
660
I
1
I
720
-1
I
740
ENERGY (cm )
Fig. IV-3.
Spontaneous ionization of Rydberg states of Na.
phenomena are shown in Fig. IV-3.
Excitation curves obtained by scanning a pulsed dye laser for increasing electric
fields constitute the data. Energy is measured from the ionization limit. The signal
is generated by ions collected following a 200 -ns delay after excitation. The disappearance of a signal indicates that the lifetime of the level has fallen below 200 ns because
of ionization by tunneling.
In regions of long tunneling lifetimes, the signal is due pri-
marily to a small amount of collisional ionization.
cate the positions of levels with
= 2.
m
In Fig. IV-3 the straight lines indi-
These levels are essentially hydrogenic and
are labeled with the parabolic quantum numbers (n, n I , n 2 , m). Unlabeled peaks are due
"A" indicates where the long-lived state (12, 6, 3, 2)
to the m = 0 and 1 levels.
Quenching phenomquenches because of mixing with the short-lived state (14, 0, 11,2).
ena of this type, which can lead to violation of the "no crossing" theorem, have been
described by Lamb. 9
n = 15 levels;
Level (12, 7, 2, 2) ionizes at "B" because of mixing with very broad
the result essentially is premature ionization.
In the absence of level
mixing, state (12, 7, 2, 2) would disappear at ~25 kV/cm.
References
1. C. Lanczos,
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Z.
Physik 68,
204 (1931).
Opt. Soc. Am. 52, 239 (1962).
2.
M. H. Rice and R. H. Good, J.
3.
J.
Hirshfelder and L. Curtiss, J.
4.
D.
Bailey, J. Hiskes, and A. Riviere, Nucl. Fusion 5, 41 (1965).
5.
A. Riviere, in Methods of Nuclear Physics (Academic Press, Inc.,
p. 208.
PR No.
118
Chem. Phys. 53,
1395 (1971).
New York, 1968),
(IV.
ATOMIC RESONANCE AND SCATTERING)
6.
R. Il'in, in Atomic Physics 3 (Plenum Press, New York, 1973),
7.
T. W. Ducas, M. G.
ters 35, 366 (1975).
8.
M. G. Littman, M. L. Zimmerman, T. W. Ducas, R. R. Freeman, and D. Kleppner,
Phys. Rev. Letters 36, 788 (1976).
9.
W. E. Lamb, Phys. Rev. 85,
PR No.
118
Littman, R. R. Freeman,
259 (1952).
p. 309.
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and D. Kleppner, Phys. Rev. Let-
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