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A NEW TECHNIQUE TO STUDY RYDBERG STATES BY MULTIPHOTON IONIZATION
SPECTROSCOPY
R. Verma, A. Chanda
To cite this version:
R. Verma, A. Chanda. A NEW TECHNIQUE TO STUDY RYDBERG STATES BY MULTIPHO-
TON IONIZATION SPECTROSCOPY. Journal de Physique Colloques, 1987, 48 (C7), pp.C7-683-
C7-685. �10.1051/jphyscol:19877166�. �jpa-00226989�
JOURNAL D E PHYSIQUE
Colloque C7, supplement au n012, Tome 48, decembre 1987
A NEW TECHNIQUE TO STUDY RYDBERG STATES BY MULTIPHOTON IONIZATION SPECTROSCOPY
R.D. VERMA and A. CHANDA
Department of Physics, University of New Brunswick, Fredericton.
NB, Canada, E3B 5A3
ABSTRACT
A new technique to study the Rydberg states of the Ba atom has been developed.
In this technique a Multiphoton Ionization signal is detected by selective excitation of the ground state ion (6s) to an excited state (6p), which results in a collimated Amplified Spontaneous Emission (ASE) signal at the 6p+5d transition of Ba'. Discrete Rydberg states, 6snL (k=0,2), as well as autoionizing Rydberg states, 5dnk (!2=0,2) and 6pnll (11=0,2) are observed by this novel but very simple method.
INTRODUCTION
Traditionally, single photon spectroscopy1 in the vacuum region has been used extensively to study the discrete as well as the autoionizing Rydberg states.
These studies were, of course, limited in their scope to the extent that from a given ground state only certain angular momentum and parity states could be excited. In recent years, laser multiphoton spectroscopy has overcome these difficulties. Currently, there are the following important modes of Multiphoton laser spectroscopy in use to study Rydberg states: ( i ) Multi-step photo2ionization
2 3
spectroscopy
,
(ii) Multiphoton-ionization mass-spectroscopy,
(iii) Multiphoton-ionization photo-electron spectroscopy4, and (iv) Optogalvanic spectroscopy 5.
Recently, Bokor et a1 showed that when two-step photo-excitation was applied 6 to excite the Ba atom from its ground state 6s to 6p3,,12p autoionizing states, an Amplified Spontaneous Emission (ASE) signals were observed at 493 nm and 650 run.
These are the transitions from the 6p,,, state to the 6s and 5d3,, states, respectively, of Ba'. They result because the autoionizing state 6p,,,nk decays (t=lO-"s) selectively to 6pll,
+
e - . The energy difference is taken up in the kinetic energy of the ejected electron, and a population inversion is created between 6pI1, and the low lying 5d3,, and 6s states of the ion.The above work of Bokor et al. provided an incentive to develop a new technique to detect the ionizing signal by selectively exciting the ion state and thereby producing a collimated ASE beam along the direction of the exciting photon.
In this paper, we describe this novel but simple and very sensitive method to study discrete as well as autoionizing Rydberg states. Basically, the method differs from the other methods mentioned earlier only in the detection of the ionization signal. Here, multiphoton ionization is observed through the ASE signal produced by selective excitation of the ground state ion via optical pumping. The technique is illustrated in detail using the Ba atom as a test case and the present results are compared with known results for Ba.
EXPERIMENTAL PROCEDURE
The experimental set up is shown in Fig. 1. Two dye lasers were pumped by an excimer laser (Lumonics). Both dye lasers delivered energy of up to 3-6 mJ per pulse with line width 0.003-0.004 nm and pulse duration of 10 ns. Two dye laser pulses were brought from opposite direction into an oven containing Ba metal in Ar (p=15 Torr) buffer and were loosely focussed by lm and 2m focal length lenses.
Pump, probe and exciting lasers, as they are called in Fig. 2. The probe laser was delayed by a few nanoseconds with respect to the pump laser. Because the probe and exciting photons belonged to the same single laser pulse, they had the same frequency. This way only two lasers were required. The probe laser intensity was cut down considerably by neutral density filters, and in later experiments this laser was used without focussing. It was important that the beams overlapped at the focal spot of the pump laser.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877166
JOURNAL DE PHYSIQUE
Pump-+
Probe
Fig. 1. The qperimental set up.
rate of 10 Hz. An optogalvanic spectrum from a commercial provided the calibration standards.
Barium vapour was produced in a steel tube, which was connected to a glass tube at both ends via a water cooled jacket.
The barium was heated by a commercial furnace (Lindberg) to a
temperature of 850°C
-
900°C. The heated zone was 120 cm long and 50 mm in diameter.
The ASE signal at 614 nm or 650 nm was
isolated from the exciting photon by a filter and detected by a monochromator- photomultiplier system.
The signal was processed by a boxcar averager at the pulse uranium H.C. lamp
RESULTS AND DISCUSSION
The underlying principle of this technique is best explained by referring to Fig. 2. A two photon resonance 6peP
of the pump laser populates the discrete Rydberg states,
6sn9. (%=0,2). A few nanoseconds later, the probe laser set at 455 run corresponding to the transition 6s+6p3,, of Ba+
excites the Ba atom frod 6snt to 6p31,n'L autoionizing states. As discussed
earlier by Tran et a12 and Mullins et a1 in this transition 7 the Rydberg electron nt simply behaves like a spectator, so that n=n' and there is only a slight adjustment in the quantum defect.
Thus the 6~nt+6p,~,n't transition has essentially the same frequency and strength as the 6~+6p,~, transition of Ba'. The
autoionizing effect is likely to cause the following decay processes: 6 ~ ~ / ~ n ' t + 6pIl2, 5d3,,,
,,,,
6s+
~-(AE). The kinetic energy, AE, of the electron corresponds to the energy difference between the Ba atom in the 6p3,,n'11 state and the final ionic state.Ba
The fact that the 6sn11+6pnk
transition frequency of the Ba Fig. 2. The process involved in the atom is nearly the same as that excitation of the spectrum of the of the 6s+6p transition Ba' discrete Rydberg states.
allowed us to use the 455 nm probe for excitation of the ion as well. Since the ionization process is very fast (t 10-12 S) and the exciting laser pulse length is long (t = s), the same pulse which acts as a probe for the atom can pump the ground state ion, 6s, up to an empty 6p3,, state. This results in a population inversion between the 6p3,, and 5d,,, states of Ba', causing an intense collimated ASE radiation at 614 nm along the path of the exciting pulse. The ASE signal was isolated and processed as described in the preceding Section.
Probe and pump intensity were reduced until we obtained a Gaussian line shape for our signal. Also, it was checked that the signal corresponded to both the pump and the probe excitation together and not to a multiple excitation of one laser.
Both lasers were linearly polarized.
When the pump laser was then scanned with the probe laser kept fixed at 455 nm, two-photon resonances at various 6snk (%=0,2) states yielded enhancements in the ASE signal providing the spectrum of the discrete Rydberg states, 6sns and 6snd, as showrrin Fig. 3.7 The present spectrum compares very well with the ionization- current spectrum
.
However, the signal detection in the present case is very simple and cheaper.When the two photon frequency of the pump laser exceeded the first ionization limit, the spectrum of the 5dnll (ll=0,2,4) autoionizing states was obtained. Here two photbn excitation drives the ground state atom to the autoionizing levels 5dnll, which decay to the 6s ground state of the Ba ion. The exciting laser fixed at 455 nm then pumps the 6s ion to the 6p3,, state causing an ASE signal at 614 nm.
Identical spectra were obtained when the above experiments were repeated but with the probe and exciting laser now set at the 493 nm line corresponding to the 6s+6plI2 transition of Baf, and the signal being detected at 650 nm, the
6p,/,+5d3~, transition line.
+
WAVELENGTH (i)Fig. 3. The spectrum of 6snll series of Ba recorded by the new technique.
REFERENCES
1. See, for example, W.R.S. Garton and K. Codling, Proc. Phys. Soc. London 75, 87, 1960; or for recent work on Ba, C.M. Brown and M.L. Ginter,
-
J. Opt. Soc. Am.68,
817, 1978.2. Tran, N.H., Pillet, P. Kachru, R. and Gallagher, T.F., Phys. Rev.A,
2,
2640, 1984.
3. Compton, R.N. and Miller, John C. AIP Conference Proc. No. 90. P.319, 1982.
4. Gallagher, T.$., AIP Conference Proc. No. 90, P.358, 1982 and also see Ref. 3.
5. Camus, P., Dieulin, M. and El Himdy, A., Phys. Rev.A,
26,
379, 1982.6. Bokor, J., Freeman, R.R. and Cooke, W.E., Phys. Rev-A,
2,
1242, 1982.7. Mullins, O.C., Yifu Zhu and 'Gallagher, T.F., Phys. Rev.A, 32, 243, 1985.
