Download Mid-Infrared High-Dispersion Molecular

Mid-Infrared High-Dispersion Molecular Spectroscopy using
Ge Immersion Grating Spectrograph (IRHS)
1
Hirahara ,
1
Hirao ,
1
Oka ,
1
Kakeue ,
2
Tokoro ,
3
Ebizuka ,
4
Kawaguchi ,
5
Masuda
Y.
T.
T. N.
Y.
H.
N.
K.
T.
1Grad. School of Environ. Studies, Nagoya University, 2Nano-Opt Institute, 3Faculty of Science, Nagoya
4
5
University, Faculty of Science, Okayama University, Instrument Development Centre, Nagoya University
Studies of Interstellar Molecules & Chemistry
More than 130 molecules have been
identified through radio, infrared (IR) and
visible spectral observations.
IR High Resolution Observations
The key species without dipole moment have
not been detected yet!
e.g. CH3+, c-C3H3+
Radio observations
・~100 species have been detected
・High spectral sensitivity and resolution
Only a few observations were reported so far …
- Orion KL, 4-25 m, IRTF (3.0 m) & IRSHELL, R = l/Dl = 25,000
(Evans et al. 1991)
Detectable only thru IR observation !!
- IRC+10216, 8-13 m, Kitt Peak Mayall telescope (4 m) & FTS, R = 100,000
(Keady & Ridgway 1993, Boyle et al. 1994)
The chemistry of dense cloud
Infrared observations
・Only ~30 species have been detected
・Molecules without dipole moment
・Precise analysis (T, N, VLSR・・・)
with rovibrational spectra
・Low spectral sensitivity and resolution
CH3+
・Low sensitivity (S/N = 20, 143 min.)
: Observable a few objects
c-C3H3+
・ Dominant noise source is black body
radiation in mid-Infrared region
￫ FTS (Fourier-transform spectrometer) is
H3+,
A key species,
has been detected using
Molecules detected by IR observations
infrared high resolution spectroscopy
H2O, CO, HF, SiO, OH, H2, C2H2, CS, CH4, NH3,
(T.R.Geballe& T.Oka,1996)
HCN, C2H4, CH, NH, HCl, SiH4, C2H, C3, C5,
CN, OCS, SiS, H3+, SH, CO2, SO2, CH3, HC3N,
• IR observation needed for
HC5N, C4H2, C6H2, C6H6, CH3C2H, CH3C4H
detailed analysis
(Nonpolar species)
IRHS
(Mid-InfraRed High dispersion
(Boyle et al., 1994)
High sensitive and resolution grating spectrometer is needed
IRHS (mid-InfraRed High dispersion Spectrograph) ; R = 40,000 @ 10 m
・A cryogenic cross-dispersed echelle spectrometer using Ge immersion grating
・A high spectral resolution in N-Band (7.5~13.5 m) : The resolving power R = 40,000
Spectrograph ) ・A broad band width : ~1 m per a rotational angle of cross-disperser
・A High sensitivity : 1Jy = 10-26 W m-2 Hz-1 (exp. time ~100 sec., S/N =10)
・For IR Nasmyth focus of NAOJ SUBARU 8.2 m telescope
Optics of IRHS
Germanium Immersion Grating
The optics consists of two sections: the fore-optics and the spectrometer optics. The fore-optics designed for IR Nasmyth
focus of NAOJ,SUBARU (F/12.5) images the telescope focal plane with magnifying power of 1.6 onto a slit mirror. The image
reflected by the slit mirror goes into slit viewer, which images the objects onto InSb detector array. The spectrometer optics
contains the Ge immersion echelle grating, the cross disperser, their collimator mirrors, the camera lenses and the Si:As IBC
(Impurity Band Conductor) detector array. To reduce thermal radiation of the instrument, all optics of IRHS is arranged on the cold
optical base plate (~30 K) of the cryostat with the diameter of 80 cm.
Collimator (F/20, f80)
Si:As
512*412
4K
・ Directly grooved onto high refractive index material
(Refractive index ‘n’ of Germanium : 4 @ 7-14 m )
・ Increases the resolving power by n-times
・ Reduces the size and the weight for the instrument
Cryostat
L
L
R=DL/l
n
Slit viewer
DL=2nL
InSb
256*256
Spectrometer optics
no longer effective
DL=2L
Conventional grating
Immersion grating
Camera lens
(F/7.7)
1140 mmf×1145 mm H
Echelle grating
Groove interval : 0.6 mm
Blaze angle : 68.75 deg.
Order : 344 – 560
Size : 30×30×72 mm
Slit Fore-optics
mirror
Echelle format
30K
Cross disperser
Immersion Grating
The Ge immersion grating has been fabricated by “ RIKEN” using ELID
(ELectrolytic In process Dressing) micro-machining method.
Focal Plane Array Detector
8 cross disperser rotational
positions provide full coverage of
7.5-13.5 m
Diameter: 800 mm
• Slit width
• Echelle grating
• Cross Disperser
0.612 arcsec (0.488 mm) at R = 50,000
Ge immersion grating
Groove interval : 0.6 mm
Blaze angle : 68.75 deg.
Order : 344-560
Short wavelength (7.5-10 m)
Groove constant : 100 grooves/mm
Blaze angle : 26.7 deg.
Long wavelength (10-13.5 m)
Groove constant : 61.97 grooves/mm
Blaze angle : 18.1 deg.
• Collimator
• Camera
• Detector
Al spherical mirror
Focal length : 500 mm (F/20)
Ge and ZnSe lenses
Focal length : 192.5 mm (F/7.7)
Spectrometer
Si:As IBC
512×412 pixels, ~30 m/pixel
Slit viewer
InSb
256×256 pixels, ~30 m /pixel
Spectrograph recorded by IRHS
Absorption of
NH3 2 band
Light Source:
Glower
Cell Length:11cm
Saturation
Time:0.83s
“net” Tinteg : 3.32sec
10.27-10.99 m region
(910 - 974 cm-1)
Si:As IBC(Impurity-Band Conductor)
Array configuration
Pixel size
Array active area
Wavelength range
Operating temperature
Well size
Quantum efficiency
Dark current
Readout noise
Si:As IBC
InSb
512×412 pixels
30×30 µm
15.4×12.4 mm
1–28 µm
4K
~1×105 electrons
> 45 %
< 1 electron/s
20 electrons
256×256 pixels
30×30 µm
7.7×7.7 mm
0.9–5 µm
35 K
~2×105 electrons
> 80 %
< 1 electron/s
25 electrons
Emission Spectra of NH3 (2)@300 ºC
Absorption Spectra of NH3 (2)
398th
399th
400th
401st
402nd
403rd
404th
405th
406th
407th
408th
409th
410th
411st
412nd
413rd
414th
415th
416th
417th
418th
419th
420th
421st
422nd
423rd
424th
425th
IRHS has been employed two focal plane array detector. One of InSb with 256*256 pixels
for slit viewer optics, and the other is Si:As IBC with 412*512 pixels for spectrometer
optics manufactured by Raytheon Co. USA.
Liq. N2 Baffle
NH3 Q(10, 10)a:
10.8859 m (918.621 cm-1 )
Lens (F12.5)
IRHS
NH3 2 mode
(+ inversion)
10.882 m
P(20)
P(20)
10.59104m P(18)
10.57105m
P(16)
10.55140m
CO2 (1, 2l, 3)=(0, 00, 1) - (1, 00, 0)
Fast Scanner Readout Direction
D(FWHM) ~8 pixels (0.053 cm-1).
~pressure broadened
N(NH3)=3 x 1018 cm-2
High S/N: ~20 for absorbance=1% in
0.166 sec. integration
(cf: ~30 secs required for the same
S/N using FT-IR & MCT detector)
10.890 m
NK=1010(s)-1010(a) : 10.8859 m
on m= 403rd (10.877 –10.911 m).
D(FWHM) ~ 7 pixels (0.047 cm-1).
Spectral Resolution: ~21,000
(focus re-adjustment under way)
NH3 Cell
(1 Torr, 300 ºC,
L=11 cm, f3.6 cm) Colour gradation indicates magnitude of transmittance
(Brighter pixel point corresponds to higher transmittance).
Wavelength : 10.27 - 10.99 m.
“Radiation Shield” (an Al Can, ~ 0 ºC)
Integration : 15 sec
Nphoton(NH3) : 105 molecules/sec