An Ultrahigh Vacuum Monochromator for Photophysics Beamline

 *P Meenakshi Raja Rao, *Saraswathy Padmanabhan, *B N Raja. Sekhar, *Aparna Shastri, *H A Khan and **A K Sinha

*Spectroscopy Division, **Centre for Design and Manufacture
BARC, Trombay. Mumbai , India – 400 085

                   The Photophysics beamline is meant for VUV (500 -2000Å) photoabsorption and fluorescence studies using synchrotron radiation from Indus-1. This medium resolution beamline uses an indigenously developed ultrahigh vacuum compatible a one-meter Seya-Namioka monochromator as the dispersing element. Seya-Namioka monochromator is being used in VUV beamlines at various synchrotron radiation facilities, due to its versatility and ease of operation. Wavelength scanning is done in the Seya-Namioka monochromator by grating rotation alone. Due to this the vacuum chamber can be small and UHV can be obtained easily. This monochromator mount was first analysed by Seya [2] and Subsequently Namioka [3,4] to optimize the parameters for its development.  In our case the monochromator will be used with vertical dispersion as the synchrotron radiation source is more divergent in the horizontal direction. Thus, the monochromator is designed [5] to operate in a vertical dispersion mode. The monochromator is developed in such a way that the grating faces downwards with horizontal slits. A brief summary of the development of the monochromator and results of the performance testing are given in this report.

2. Developmental details:

                     Wavelength scanning in the Seya-Namioka mount is achieved through a Sine-drive mechanism, which basically converts the rotation of the grating into linear motion. It consists of three parts:

1.      The UHV feed through connected to the grating mount.

2.      The ball screw with the guide driven by a stepper motor.

3.      The lever, which connects the feed through to the guide.

                    If q is the angle made by the lever with the normal to the screw,

                        Sinq = Displacement along the screw/lever length.

                     The lever length in our monochromator is fixed at 136 mm. An encoder with a digital readout connected to the end of the screw gives the wavelength reading. The design parameters used in the fabrication of the monochromator are listed in table 1.

Table 1

1.Vacuum requirement                        - 1 X 10^-9 mbar

2.Pumping System                                - 270l/s SIP, backed by 140 l/s TMP

3. Monochromator Chamber

 a) Material                                           - SS 304 L

 b) Surface finish                                  - 0.1 m, electropolished

 c) Dimensions                                      - ID = 320 mm,- thickness = 5 mm , height = 250 mm

4) Arm lengths 

      1. Entrance arm (-r)                      - 818.3 mm

       2. Exit Arm(r^|)                              - 818.3 mm

5.Grating                       

 a) Radius of curvature                      -  998.3 mm

 b) Coating                                          - Gold

 c) Type                                               - Concave, Holographic

 d) Groove density                              - 2400 l/mm              

 e) Blaze wavelength                          - 600 Å

 f) Dimensions                                    - 63.5 mm f, 12.5 mm thick

                                                             - 50 mm X 30 mm ruled area.

6) Entrance and Exit slits                 - UHV Compatible variable slit( 5 – 3000 m)

7) Angle subtended at the grating   - 70^o15’

8) Grating Mount                             - Kinematic mount with provisions to adjust   

                                                              grating in X, Y, Z directions as well as tilt.

9) Wavelength Scanning                  - Rotation of the grating by a UHV feedthrough

                                                              coupled to a sine drive mechanism.

10) Feedthroughs                             - a) UHV compatible Wilson sealed feed through

- b) Bellows Sealed feed through.

11) Wavelength reproducibility     - ± 1Å(first order)

12) Resolution (first order)             - 2.5 Å using a 1200 l/mm grating

13) Detection inVUV                       - With a sodium salicylate coated photomultiplier.

14) Data acquisition                        - PC interfaced.

Based on the above-mentioned design parameters the monochromator was fabricated and tested. Figure1 shows a photograph of the instrument.

Fig.1. One- meter Seya-Namioka Monochromator Assembly.

3. Performance Testing:

                   Preliminary alignment was done at atmospheric pressure using UV-Visible sources. First the focus of the system was tested using a concave mirror with radius of curvature one meter and dimensions similar to that of the original grating. Using a tungsten lamp and the mirror the zero order reflected image at the exit slit was viewed and the arm lengths of the monochromator adjusted for the best image. Replacing the mirror with a grating a similar exercise was carried out to check focus.. For wavelength scanning testing, a concave grating with 1200 l/mm and other dimensions similar to that of the 2400 l/mm grating was used. A mercury pen lamp giving intense lines in the UV-Visible region (2500 – 6000Å) was used as the source. The spectrum of this source was scanned in the wavelength region 2500 - 6000Å. A large number of scans were taken with the wavelength being read out from the encoder in order to check the reproducibility. The spectral lines were found to be reproducible to ± 1Å. In order to test the spectral resolution of monochromator the hollow cathode lamps emitting sharp spectral lines were used. Figure.2 shows the spectrum of Fe hollow cathode in the region 2300 – 2700Å recorded with a slit width of 100 m. It can be seen that spectral lines separated by 2.5 Å are well resolved. (eg.2462.7 Å & 2465.2 Å). After performing resolution testing, using a 150 l/s TMP as the fore pump and an SIP of 270 l/s, vacuum of the order of 10^-9 mbar has been obtained in this monochromator.

Fig.2 Spectrum of Fe hollow cathode lamp

                   Recording the emission spectrum of the CO molecule tested the performance of the monochromator in the VUV region. For this experiment a LiF window was used at the entrance slit and a sodium salicylate coated quartz window was mounted at the exit slit. A PMT was used to detect the signal. The fourth positive bands of CO were excited using a microwave discharge in a discharge tube evacuated to 10^-2 mbar by a rotary pump and coupled to the entrance slit of the monochromator. The spectrum obtained in the 1500 –2000 Å region is shown in figure 3.

 Fig.3 Band Spectrum of CO in the VUV region.

 4.Conclusions:

                    A meter UHV monochromator in the Seya-Namioka mount was developed indigenously. The monochromator has been tested for its performance at atmospheric pressure as well as under UHV. The performance testing shows that it has a wavelength reproducibility of ± 1Å, a resolution of 2.5 Å with a 1200g/mm grating and performs well in the VUV as demonstrated by the recorded CO spectrum. At present this monochromator is a part of the recently commissioned photophysics beamline.

5.Acknowledgements:

                  The authors thank Dr. N.C. Das, Dr. T.K.Balasubramanian and Dr. A.P.Roy for useful discussions. We also acknowledge the help rendered by Spectroscopy division workshop in assembling and testing the monochromator.

6.References:

1.      ”Techniques of Vacuum Ultra Violet Spectroscopy”, James A.R. Samson, John Wiley & Sons, New York (1967).

2.      M.Seya, Sci.Light 2 8 (1952).

3.      T. Namioka, Sc.Light 3, 15 (1954)

4.      T.Namioka, J.Opt.Soc.Am 49 , 959 (1959)

5.      ”Image Evaluation of One Meter Seya-Namioka Monochromator at Indus-1 synchrotron Radiation    Source by Ray Tracing”, N.C. Das, B.N. Rajasekhar and D.V. Udupa, BARC/1994/E/044 (1994).