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