c. Dye lasers and ionization detection

Dans le document The DART-Europe E-theses Portal (Page 163-170)

Part II - Abstract

II. PREPARATION AND MANIPULATION OF COLD MOLECULES

II.3. VIBRATIONAL COOLING WITH BROADBAND LIGHT

II.3.2. c. Dye lasers and ionization detection

The detection of the molecules formed by photoassociation is done by ionization via various REMPI schemes. In order to calculate the efficiency of the ionization process, we have to take into consideration the spacial overlap of the photoassociation laser with the atoms, the molecule's free fall (they are not trapped) and the overlap with the ionization laser. As we find in [Comp99], for a position of the ionization laser displaced by the MOT by d and h (horizontally and vertically), for a time t we have for the number of produced ions Nions

Nionst=RmolNatPA

radiation and in red the shaped radiation in the exit of the 4-f line.

r=

122ratrat22PA2 PA2 4ion2 (3.5)

where ωPA and ωion are the waists of the photoassociation and the ionization lasers respectively and with velocity distribution of the molecular cloud

=

kBTmol/mCs2 (3.6)

The terms Rmol and NatPA correspond to the molecule formation rate and to the number of atoms in the zone which overlaps with the photoassociation laser. The factor η corresponds at the efficiency of the Multi-Channel Plate (MCP) ions detector multiplied with the efficiency of the ionization process

=MCPion (3.7)

where ηion is the efficiency of the ionization process. Our experimental parameters are: σ = 300 µm, ωPA ~ 170 µm and ωion ~ 300 µm the size of the photoassociation and ionization lasers, while the time for which detection of molecules is possible is between 1 and 16 ms, with the optimal timing to be in t ~ 10 ms. With the detection laser crossing the MOT approximately at a distance of 200 µm from its center the integral 1.16 gives 0.15, which means that only 15% of the molecules are in the detection zone. If we consider that 30% of the formed ion reach the MCP detector we take ηMCP

~0.27 and for the total detection efficiency something more than 4%. That means that when 100 ions are detected, the total number of molecules formed in the MOT is around 2500.

The MCP detector is mounted and power supplied as shown in Fig. II.3.2.c.1. Our MCP detector is consisted of two parallel MCR arrays which are power supplied with a total voltage of -1700 V in the front side of the MCP, with -810 V in the middle and with -81 V in the back. The circuit is placed in a Cooper box in order to minimize the external noises, while the connecting cable length is minimized in order to increase the signal to noise ratio.

The signal is amplified by an RF amplifier and is inserted to one of the eight channels of an 1GHz LeCroy Waverunner 104xi digital oscilloscope. The digital oscilloscope is also connected to an Ansgtrom WS8 optical fiber lambdameter. The lambda meter detects the wavelength of either the photoassociation or the ionization laser. The digital oscilloscope is equipped with LabVIEW, and a software permits the simultaneous registration of the ion signal and the corresponding wavelength. By scanning the photoassociation or the ionization laser we can acquire photoassociation or ionization spectra of up to one million points (measurements).

Fig. II.3.2.c.1: Set up of the MCP ion detector. On the left, the under-vacuum assembly which is consisted of the two MCP stages and the collector. On the right, the power supply diagram. After collected by the

collector, the signal is amplified before being read in the output.

The laser used for the molecule ionization is a Nd:Yag pumped, home made dye laser which delivers pulses of ~10 ns with a spectral width of ~3 GHz. The wavelength can be tuned in a large range of values, depending on the choice of the laser dye used. The usual detection frequencies are

~16000 cm-1, which corresponds to the DCM dye (wavelength range 14500 – 16450 cm-1). In this wavelength range we have the possibility to detect both the X1Σg+ singlet and the a3Σu+ triplet state molecules, as shown in Fig. II.3.2.c.2.

For the case of the X1Σg+ singlet state, detection is done via the C1Πu state which is shown with the blue line in part (a). This is a standard ionization scheme reported in [Raa82]. The states shown with the dotted green and red lines perturb the C1Πu state, with result for the states with vibration v > 12 to be pre-dissociated and not to be detected by this ionization scheme. As far as the a3Σu+ triplet state, which is shown in part (b), detection can take place by both the 0g and 1g states.

By this detection scheme, three major photoassociation lines are observed in a detuning of -2.1, -6.1 and -6.4 cm-1 from the 6s+6p3/2 dissociation limit. Due to the high number of molecules produced by these photoassociation lines, they are called 'giant lines' [Vata99]. The possibility to detect both the X1Σg+ singlet and the a3Σu+ triplet states with the same dye is very useful. The giant rates give an easy way to produce a large molecular signal, with the position of the photoassociation and the detection lasers can be experimentally optimized. Then the detection can switch to the X1Σg+ singlet state, which is much more difficult to detect since the signal is smaller.

Finally, in Fig. II.3.2.c.3 we show the modification that permitted the generation of broadband laser radiation by a different dye laser (Continuum dye laser). Originally, the laser cavity contains a series of four anamorphic prisms and a 2400 lines/mm optical grating as shown in part (a). The grating permits the selection of a part of the laser's bandwidth and the laser radiation generated this way has a spectral bandwidth of 0.2 cm-1. After the modification, which consisted of removing the prisms and the grating and substituting them by a single prism (and a mirror), the laser bandwidth is increased to ~25cm-1.

Fig. II.3.2.c.2: Usual ionization scheme for the detection of the Cs dimers in the X1Σg+ singlet (a) and the a3Σu+ triplet states (b) in ~16000 cm-1 and ~14000 cm-1 respectively.

II.3.3. Conclusion

In this chapter I tried, on the one hand, to describe the vibrational cooling technique and the line of experiments that lead to its demonstration, and on the other, to present the experimental set up usually used in the Cold Molecule laboratory, with economy and clarity. After a small introduction and a summary of the most important cold molecule technique in the previous chapters, I describe the technique, in the study of which, this part of my thesis is reporting. This is done with an emphasis on the operating principle in which the technique is based, so that the various extensions and generalizations can be more easily discussed. Then, I describe the spectroscopic studies that lead to the preparation of tightly bound Cs dimers and the demonstration of the vibrational cooling technique in Laboratoire Aimé Cotton. Finally, I describe the basic components of the experimental set up used in the initial experiment and in the following of this study.

Fig. II.3.2.c.3: Modification in the Continuum dye laser cavity for broadband detection of molecules. In (a) optical cavity before and in (b) after the modification.

CHAPTER II4: MOLECULAR POPULATION TRANSFER

TO A GIVEN VIBRATIONAL STATE

II.4. MOLECULAR POPULATION TRANSFER TO A GIVEN

Dans le document The DART-Europe E-theses Portal (Page 163-170)