Towards High Resolution Cavity Enhanced Spectroscopy with Fast ion Beams
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Transcript of Towards High Resolution Cavity Enhanced Spectroscopy with Fast ion Beams
Towards High Resolution Cavity Enhanced Spectroscopy with Fast ion Beams
Andrew Mills, Brian Siller, Manori Perera, Holger Kreckel, Ben McCall
High-Resolution Techniques
Oka, Saykally Maier, Nesbitt
Ion-neutral discrimination
Low rotational temperature
Narrow linewidth
Compatible with cavity-enhanced spectroscopy
Mass spectrometry of laser-probed ions
Velocity
Modulation
Supersonic
ExpansionSCRIBES
Hollow
Cathode
Hirota, Amano
High ion column density
SensitiveCooledResolvedIonBEamSpectroscopy
Fast Ion Beam Spectroscopy
• Pioneered by Saykally group, late 1980s• Five (known) ions studied
– HF+, HN2+, HCO+, H3O+, NH4
+
• Abandoned in early 1990s• Struggles:
– ions still at high temperature– low ion column density– problems with infrared laser
• New approaches:– supersonic ion source– high-sensitivity spectroscopy– difference frequency laser
Coe et al., JCP 90, 3893 (1989)
velocitymodulation
fast ionbeamHF+
SCRIBES OSU 2009
Cold CathodeSource
CylindricalBender
Beam Modulated Time of FlightMass Spectrometer Not at Same Time
as Spectroscopy
SCRIBES OSU 2009 differential pumping
movable apertures
beam modulatedtime of flight(removed forspectroscopy)
924.410924.409924.408924.407924.406924.405
1.0
0.8
0.6
0.4
0.2
20
15
10
5
0
-5
-10
x10-9
min ~ 4.5 x 10-10 cm-1
Wavelength (nm)
Abs
orba
nce
(cm
-1)
x10-9
Cavity Ringdown Spectroscopy• “CW” cavity ringdown with Ti:Sapphire laser• Beam modulation (on/off) reduces long-term drift
frequencyvelocityVbeam
SCRIBES @ OSU 2010
• Removed ion beam machine from laser table– reduce vibration from pumps
• Higher sensitivity spectroscopy– separate cavity from ion beam machine– cavity-locked spectroscopy
• Simultaneous laser and mass spectroscopy– verify mass distribution– verify absolute beam energy (Doppler shift)– fast high voltage probe
SCRIBES, OSU 2010
separate support structure on wheels
external cavity on floating optics bench
Cavity-Enhanced Velocity Modulation
Lockbox
PZT
EOM
Ti:Sapph
Lock-in Amplifie
r
QWPAOM
fast
slow
PDH: Drever, Hall, Kowalski, Hough, Ford, Munley, Applied Physics B-31 (1983) 97–105.
Excursion to test Spectroscopy FD2
Mass Spectrometry: Mass Distribution
N2+
N+•Mass Spectrometer calibrated with an air discharge.
•Nitrogen discharge shows mostly N2
+, some N+.
•Ion current should represent N2
+ current.
Mass Spectrometry: Energy Shift•Measured time over many measurements
•Day to day variation
•Does not match the output of the power supply
(3900 vs 3817)
3.4 V, 77 MHzShort term (several minutes)
1 V, 46 MHzLong term (several hours)
June 18, 2010 Scans
• Measured float voltage with fast high voltage probe 3900
• Beam energy measured MS 3900 V
• Laser scanned over 3880-3960V beam energy for the Q11(14) N2
+ blue shifted line, modulated at ± 10 Volts.
• Expected S/N ~(9-22).• Still troubleshooting this very
recent addition of spectroscopy to the ion beam
Conclusions• A laser-table independent ion beam instrument
has been constructed with well collimated ion beam collinear to a laser beam.
• The ion beam allows for simultaneous monitoring of the beam energy prior to spectroscopy.
• The output of the power supply monitor voltage is not accurate enough and the mass spectrometer must be used to determine the beam energy.
• Preliminary scans have been performed, and we are continuing to search for a signal.
Acknowledgments
Air Force Young Investigator AwardFA 9550-07-1-0128
Visit us at: http://bjm.scs.uiuc.edu
NASA LaboratoryAstrophysics
NSF Chemistry, Physics, Astronomy
Dreyfus New Faculty, Teacher-Scholar
Awards
Packard Fellowship
CottrellScholarship
SloanFellowship