Production of Molecular Ions Using a Hollow-Cathode Spectrometer

Trevor Cross, Nadine Wehres, Mary Radhuber, Anne Carroll, Susanna Widicus Weaver

Department of Chemistry, Emory University, Atlanta, GA, USA

Motivation for Laboratory Search of Ions in ISM

• Tracers of chemical and physical conditions in ISM (Herbst & van Dishoeck 2009)

• Important intermediates in chemical networks

• New telescopes coming online

Why a Hollow Cathode?

• Efficient creation of protonated species (Gabrys 1995)

• Long pathlength

• Access to highly excited species

Schematic

Detector

Lock-in Amplifier

Synthesizer

N2H+

To Pump

Sample Input

LN2 Cooling

Recirculating

Chiller

HV

THz Source

LensLensDesign based on

Amano Design based on Gabrys et al. J. Phys. Chem. 99 (42)

(1995)

Further Specifications

• Copper cell with cooling coils

• Stainless Steel anode with cooling lines

• Liquid nitrogen, water or ethylene glycol cooling

• Pressure: 50 mTorr of sample gas in argon

• Tunable HV power supply (max. 2000 V), (typical instrument settings: 300-500 V and 180 mA)

Target Molecules

• N2H+ for benchmark and proof of concept

• N2D+ up to 1 THz

• H5+ isotopologues

Background of Target molecules N2H+ and N2D+

• N2H+ first observed in the ISM. (Turner 1974)

• First experimental detection of both N2D+ and N2H+ (Saykally 1976)

• N2H+ fully characterized up to 2 THz (Amano 2005, refs. therein)

• N2D+ characterized up to J’-J”=9-8 at 700 GHz

• Important tracers (Herbst 1989, Loren 1995, Lepp 1984)

N2H+ Detections

• Detected N2H+ transitions between 300 GHz up to 1 THz

• Conditions for experiment• 5 sccm H2, 5 sccm N2, 40 sccm Ar

• Proof of concept for experiment

4000

2000

0

-2000

Inte

nsi

ty [

a.u

.]

745216745214745212745210745208745206745204

1000

500

0

-500

558974558972558970558968558966558964558962558960

2000

1000

0

-1000

931392931390931388931386931384931382931380

4000

2000

0

-2000

838314838312838310838308838306838304838302838300

20

10

0

372680372678372676372674372672372670372668372666

Frequency [MHz]

400200

0-200-400

465832465830465828465826465824465822465820465818

4000

2000

0

-2000

Inte

nsi

ty [

a.u

.]

745216745214745212745210745208745206745204

1000

500

0

-500

558974558972558970558968558966558964558962558960

2000

1000

0

-1000

931392931390931388931386931384931382931380

4000

2000

0

-2000

838314838312838310838308838306838304838302838300

20

10

0

372680372678372676372674372672372670372668372666

Frequency [MHz]

400200

0-200-400

465832465830465828465826465824465822465820465818

N2H+

Detections

N2D+ Detections

• N2D+ detected in the range of 300 GHz to 1 THz.

• Same conditions as N2H+

• Higher frequency transitions were calculated by JPL/CDMS

• Confirmation of predicted transitions between 700-1000GHz

• J’-J” = 10-9 through J’-J” = 13-12

12000

8000

4000

0

-4000

770855770850770845770840

Freuqency [MHz]

4000

2000

0

-2000

847885847880847875847870

2000

1000

0

-1000

Inte

nsity

[a.u

.]

924895924890924885924880

6000

4000

2000

0

-2000

1001890100188510018801001875

CDMS

JPL

N2D+

Detections

Significance of N2D+ Detections

• Observers can unambiguously identify these new N2D+ lines relying on experimental detections.

• Significant difference from predictions

• Refined molecular constants

New Molecular Constants and FitTransitions Observed Calculated JPL CDMS

10-9 770848.990 (40)

770849.030 770850.6(1.2) 770849.034(32)

11-10 847877.060(40)

847877.090 847879.3(1.6) 847877.158(44)

12-11 924888.980(40)

924888.910 924892.0(2.2) 924889.065(60)

13-12 1001883.060(40)

1001883.012 1001887.0(2.8) 1001883.282(78)

Parameters This Work Dore et al 2004

B (MHz) 38554.75529(90) 38554.7523(26)

D (kHz) 61.5185(46) 61.552(47)

eQq1 -8.488(13) -5.6587(42)

eQq2 -2.053(24) -1.1713(73)

C(N1) 3.87(18) x 10-3 4.80(84) x 10-3

C(N2) 5.57(24) x 10-3 7.6(11) x 10-3

Future and Work in progress

• H2D+ and D2H+ available from the CDMS and JPL databases

• H5+ isotopologues measurements

• Other weakly bound ions or radicals

H5+ Isotopologues

• Highly fluxional and weakly bound cluster

• Molecular interaction

• Three isotopologues with dipole moments:

• H3D2+ , H2D3

+ , and H4D+

McGuire et al. 2011

H5+ Isotopologues Boltzmann Peak

• Warmer excited states more accessible

• Ideal peak for spectral range

McGuire et al. 2011

20K 300K

Acknowledgements

• This work is supported by NSF CAREER Award CHE-1150492.

• Thanks to the Widicus Weaver group