Doppler free spectra

Fig. 2. Doppler-free spectra of the 15 — 2S two-photon transition (F = 1 —> F = 1) in atomic hydrogen, a) Spectra for three different nozzle temperatures and no delay time, b) Time resolved spectrum (nozzle temperature 6.5 K). This plot gives the 2S count rate as a function of the absolute optical frequency for different delay times. The inset shows the spectra with longer delay times on a magnified scale...

The idea of using a train of coherent pulses for the observation of the IS to 2S transition in hydrogen was first suggested by BAKLANOV et al. in 1976 [10]. The observation of Doppler-free spectra using a coherent pulse train from a synchronously pumpecl dye laser was demonstrated by ECKSTEIN et.al [11]. 

In the following I shall discuss a number of recent laser experiments on two-body systems namely, hydrogen, positron urn and muonic atoms (u He). In describing these experiments I will be introducing several applications of Doppler-free laser spectroscopy and of frequency-doubled tunable radiation. I will spend first a little time on the theory of the hydrogen atom, contrasting it with that of positronium and muonium. I will then make one or two remarks about frequency calibration of Doppler-free spectra and then consider in some detail laser experiments performed in Oxford and Stanford on hydrogen. 

The application of two-photon spectroscopy to molecules has brought a wealth of new insight to excited molecular states. One example is the two-photon excitation of CO in the fourth positive system A TJ Ug and of N2 in the Lyman-Birge-Hopfield system with a narrow-band pulsed frequency-doubled dye laser. Doppler-free spectra of states with excitation energies between 8-12 eV can be measured with this technique [253]. 

Resonant two-step excitation by means of co- or counterpropagating cw dye laser beams results in Doppler-free spectra due to absorption line narrowing. Starting from the atomic ground state Ig), certain velocity ensembles, Doppler-tuned into resonance, are excited by one of the laser beams to the intermediate level i). When the second laser is scanned across... 

Doppler-free spectra of 6snp Pi (10 n 40) Rydberg states of barium (see Figures 12 and 13) have been recorded by employing thermionic detection after resonant two-step excitation via quadrupole (65 5d D2)- dipole (5d D2 65np Pi) transitions (cf. Section 2.2.2). In Figure 55 the frequencies of the hyperfine components iy = 1/2, 3/2, and 5/2 of Ba relative to Ba have been plotted versus the principal quantum number n. Apart from the correction for the normal mass shift,... 

In two-photon spectroscopy it is possible to record Doppler-free spectra without any need for velocity selection by excitation with two counterpropagating laser beams whose first order Doppler shifts cancel. 

The Doppler-sensitive line gives a second clear signature for Bose-Einstein condensation. Because the lowest energy state is the lowest momentum state, the condensate appears as a relatively narrow peak at the center of the Gaussian spectrum. Its width is given by the cold collision frequency shift and is the same as in the case of Doppler free spectrum. 

Figure 3. Doppler-free spectrum of Balmer-a in tritium. (The resolved splitting in the 2si/2 SPi/z/SPa/z transitions is due mainly to the 177 MHz hfs of the 2s state.)...

The Fourier analysis of the time-dependent signal (7.27a-7.27b) yields a Doppler-free spectrum /( ), from which the energy spacing A as well as the width y of the two levels ) can be determined, even if A is smaller than the Doppler width of the detected fluorescence (Fig. 7.9c). Quantum-beat spectroscopy therefore allows Doppler-free resolution [868]. 

Figure 4 Principle of Doppler-free two-photon excitation (A) resonant two-photon excitation steps (B) frequencies of coun-terpropagating laser beams, as seen by atoms of the velocity component v (C) population densities of competing levels according to a well-defined class of velocity of analyte atoms (D) resulting Doppler-free spectrum.

Figure 15. Doppler-free spectrum of the 6sl5p Pj state of Yb obtained by two-photon-resonant three-photon absorption. (Taken from Ref. 45.)...

Figure 49. Doppler-free spectrum of the 65164 D2 Rydberg state of Ba, obtained by optical-optical double resonance. The hyperfine components of have been labeled...

For illustration Fig. 5.61 shows a Doppler-free spectrum of naphthalene CioHg recorded together with frequency markers from a stabilized etalon and an 12-spectrum providing reference lines [392]. 

Figure5.61 Frequency markers from an etalon withrtJ = 50 cm, Doppler-limited and Doppler-free lines of I2 as reference spectrum and a section of the Doppler-free spectrum of the naphthalene molecule, taken in a cell with about 5 mbar [392]...

Fig.9.49. Doppler-free spectrum for high-lying Rydberg states in Ba [9.192]... 

In saturation spectroscopy, a monochromatic laser beam labels a group of atoms within a narrow range of axial velocities through excitation or optical pumping, and a Doppler-free spectrum of these selected atoms is observed with a second, counterpropagating beam. 

Fig. 2 Top Balmer spectrum of atomic hydrogen. Center Doppler profile of the Balmer-a line at room temperature and theoretical fine structure components. Bottom Doppler-free spectrum of Balmer-a, recorded by saturated absorption spectroscopy with a pulsed dye laser.

The data recorded as the laser frequency is scanned consists of the fluorscence signal from the PMT, a Doppler-free I2 spectrum and frequency markers from the etalon. The etalon provides a calibration of the frequency scan. The Doppler-free I2 spectra provides an absolute frequency reference used to correct for small laser frequency drifts, separator voltage drifts and to determine the absolute acceleration voltage of the separator for the Doppler shift corrections. We are thus able to record data over long periods of time, e.g. 3 hours, and maintain a reasonable resolution of 100 MHz. Some of the first online data recorded with this system is shown in Figure 2. The overall detection efficiency has been measured to be 1/1000, i.e. one detected photon per 1000 atoms, for the largest transition in the nuclear spin 1/2 isotopes. 

When the transition temperature is achieved, a finite fraction of the atoms fall into the lowest energy quantum state of the trap. The spatial extent of the condensate is much smaller than the thermal radius of the cloud. Only a small fraction of the atoms are required to create a narrow region of very high density at the bottom of the trap. This high density region is readily observed because of its large cold collision frequency shift. The spectrum arising from the condensate can be seen in (Fig. 5), red-shifted up to 0.5 MHz from the Doppler free line. 

Fig. 5. Composite 15-2,S two-photon spectrum of trapped hydrogen after condensation. o-spectrum of sample without a condensate -spectrum emphasizing features due to a condensate. The high density in the condensate shifts a portion of the Doppler-free line to the red. The condensate s narrow momentum distribution gives rise to a similar feature near the center of the Doppler-sensitive line...

Doppler-free saturation spectrum of 130Te2 in the region of Balmer p in hydrogen shown as a scan over 2.5 GHz. Also shown is an inserted spectrum of the hydrogen IS to 2S transition at one quarter of the transition frequency. 

Fig 6 Two experimental arrangements for demonstrating the quality of an FM spectrum a) sum frequency generation in a nonlinear crystal followed by mode analysis of the generated ultraviolet b) Doppler-free two-photon spectroscopy. 

Fig 8 a) Doppler-free two-photon spectrum of the sodium 3S to 4D transition taken with a single frequency dye laser, b) The same transition observed using an FM dye laser. 

Fig. 1. Doppler-free saturation spectrum of the hydrogen Balmer-a line compared to theoretical fine structure and Doppler profile at room temperature [4].

The ground electronic state of 139La160 is X2S+ audits electronic spectrum involving the excited B2Y,1 has been studied by Doppler-free laser-induced fluorescence by Bacis, Collomb and Bessis [85] and by Bernard and Sibai [86]. Both states have therefore been well characterised and the system is ideal for radiofrequency/optical double resonance, as described by Childs, Goodman, Goodman and Young [87]. They used a collimated molecular beam, with the laser pump/probe technique described elsewhere in this chapter. 

Figure 11.43. Section of the Doppler-free laser-induced fluorescence spectrum of LaO, arising from the B 2E+(u = 0)—> X 2E+ (v = 1) electronic transition [87], Four different rotational components are present, one of which is marked, with the lower state N value (30) being given in the brackets. The region of the electronic spectrum scanned is 5866.75 to 5866.80 A.

The use of electric field modulation in recording this spectrum greatly enhances the peak heights of the sharp Doppler-free Lamb dips relative... 

Fig. 2.33 Doppler-free two-photon spectrum of the Na atom (a) level scheme of the 35 55 and 35 4D transitions (b) 35 55 transition for AF = 0 with resolved hyperfine structure [245]...

Fig. 2.37 Section of the Doppler-free two-photon excitation spectrum of the 14j Qq bandofC6H6 [255]...

The optical pumping of the molecules is now performed with a separate laser beam, which is sent through the sample cell anticollinearly to the probe beam for V-type OODR, but collinearly for A-type OODR (Fig. 5.34). In order to keep the pump transition on the wanted selected transition, at first an ordinary Doppler-free polarization spectrum of the pump laser must be recorded. Therefore, the pump laser beam is split into a pump and a probe beam, and the spectrum is recorded while laser L2 is switched off. Now the pump laser is stabilized onto the wanted transition and the second (weak) probe laser L2 is simultaneously sent through the cell. 

---