josé luis doménech molecular physics department instituto ...€¦ · josé luis doménech...

Laser Spectroscopy of ions of astrophysical interest

José Luis Doménech Molecular Physics Department

Instituto de Estructura de la Materia CSIC

Jose Luis Domenech. Madrid 24-26 Marzo 2015. Curso de Iniciacion a la Investigación en Estructura de la Materia

OUTLINE: • Motivation.

• Experimental set-up.

• Infrared spectroscopy of NH3D+ and its identification in

space.

• Spectroscopy of 36ArH+ and 38ArH+.


MOTIVATION

• The interstelar medium is cold and dilute. How can complex molecules be formed?

• Molecular ions are key intermediates for the formation of complex molecules in space. The only reactions occurring in the gas phase are binary (two

reactants), exoergic, and indirect (no entrance barrier)). Ion– molecule reactions are known to have no entrance barrier.

• In space molecular ions are abundant given the long time between

collisions and the excitation by UV photons and cosmic Rays.

• Laboratory spectroscopy is essential to the interpretation of ISM spectra. But the ion density in laboratory plasmas is really low: ~109-1010 cm-3

• High sensitivity, accuracy and spectral resolution are necessary in the

laboratory.


NH3D+: MOTIVATION

• The Ammonium Ion (NH4+) is the starting point to form

Ammonia (NH3) and amine prebiotic molecules in Space

• NH4+ has tetrahedral symmetry, hence no permanent electric

dipole moment, and no pure rotation spectrum, so it can not be observed with radio-telescopes.

• NH3D+ has C3v symmetry and a small dipole electric dipole moment (0.26 D), so it should be usable as a tracer of NH4

+ in the insterstellar medium.

• There is no laboratory rotational spectrum (mm-wave) of NH3D+


NH3D+: MOTIVATION • A strong unidentified line at

262816.7 MHz in Orion-IRc2 1

• From the analysis of the 4

infrared band of NH3D+,

Nakanaga & Amano2 predicted the 10-00 transition at 262807±9 MHz (±3σ)

• No match with other known species in Orion.

• Is U262816.7 NH3D+ ?

• With yet 3500 unidentified lines in the Orion survey… too risky.

1 Tercero, B., Cernicharo, J., Pardo, J. R., & Goicoechea, J. R., 2010, A&A, 517, 96 2 Nakanaga, T., & Amano, T., 1986, Can. J. Phys., 64, 1356

N&A (1986)


Observations towards B1-b

• Narrow and isolated feature (no more lines in 3.8 GHz) • Kinetic temperature is ~ 12 K, so the carrier has to be a light species • NH3, 15NH3, NH2D, 15NH2D, ND2H, ND3 have been detected in B1-b,

plus NNH+, NND+, 15NNH+, N15NH+

• More laboratory data are needed!!

26282 26281

262816.73 MHz

/MHz


IRAM 30m Pico Veleta

(1) (2) (3)

0(,)(,)(,)(,)(,)(,)(,)...PrtErtErtErtErtErtErt

IR generation by difference frequency mixing

LiNbO3

33 1 122()o e onnTn

1

2

3

(2)

3120 3121 2 i ijk j kjk

P gd EE

3 1 23 1 2k k k


• I2 locked Ar+ laser & Ring dye laser

• Mixing in LiNbO3 (DFG)

• Spectral purity: ~1:108 (2 MHz)

• Tunability 2.2 – 4.2 µm

(2300-4500 cm-1)

• 1.2 cm-1 continuous scan

• Sensitivity: ~1:104 (in transmission)

• 10 µW vs. 10-5 µW NEP of InSb

• First setup: Alan Pine (JOSA1976)

• Accuracy: ~3 MHz (1σ)

• Wavemeter for the dye laser.

• At each datapoint in the spectrum, we have an ‘instantaneous’ wavemeter reading. The wavemeter is calibrated with the Ar + laser.

• We can average a lot of scans.

Difference frequency spectrometer


The discharge cell

• Hollow-cathode discharge • Design Foster & McKellar • 380 V, 200 mA • White cell ~9 m inside the

cathode • Amplitude modulated at 5.5 kHz • IR modulated at 14.2 kHz • Lock-in detection at 19.7 kHz

Precursors: NH3, D2, 2:3 Total pressure 0.34 mbar


This work Nakanaga & Amano (1986)

Lines marked with * belong to NH4+

Rather similar S/N

Ion concentration ~1010 cm-3 ~10-9 bar Jose Luis Domenech. Madrid 24-26 Marzo 2015. Curso de Iniciacion a la Investigación en Estructura de la Materia

10-00 pure rotation transition (radioastronomy observation)

v4=1 is an E state (asymmetric N-H stretch) «Perpendicular» type band of a prolate rotor Coriolis splitting and k-type doubling ΔJ=0, ±1; ΔK= ±1


IR transitions

NH4+ Schafer, Saykally, Robiette

1984, JChemPhys 80, 3969

This work

Both this work

and N&A

Simulations and fits have been done with PGOPHER . (Colin Western, University of Bristol, http://pgopher.chm.bris.ac.uk)


http://pgopher.chm.bris.ac.uk/



NH4+ Schafer, Saykally, Robiette

1984, JChemPhys 80, 3969

This work

Both this work

and N&A


We have recorded 114 transitions between 3268.4 and 3414.7 cm-1

Finally 76 have been included in the fit (vs. 61 in N&A) Lines not included: J,K >8; or show interference from NH4

+, NH3; or are too broad

A’, A’’, DK’, DK’’, Cannot be determined independently. Constraining A’’, DK’’ affects 0, , A’, DK’ s.d. of the fit 5×10-4 cm-1

(10 − 00) = 2B’’-4D’’

= 262816.8 MHz; σ = 1.8 MHz

vs 262816.73 MHz in the radio astronomy observations


N&A (1986)

• Our frequencies are good to 10 MHz (3σ) • Our Ar+ laser is locked to an I2 line known with 0.1 MHz accuracy • We have measured some more lines than in N&A work • All statistical parameters are somewhat better

• We propose 262817±6 (±3σ) as the frequency of the 10-00 rotational transition of

NH3D+, supporting the assignment of emissions in Orion IRc2 and B1b to NH3D+

• Published in Astrophysical Journal Letters; Cernicharo et al. 771 L10 (2013). and Doménech et al. 771 L11 (2013).

º±3σ

NH3D+


http://dx.doi.org/10.1088/2041-8205/771/1/L11

http://dx.doi.org/10.1088/2041-8205/771/1/L11

http://dx.doi.org/10.1088/2041-8205/771/1/L11

http://dx.doi.org/10.1088/2041-8205/771/1/L11

http://dx.doi.org/10.1088/2041-8205/771/1/L11

http://dx.doi.org/10.1088/2041-8205/771/1/L11

http://dx.doi.org/10.1088/2041-8205/771/1/L11

Crab Nebula image (Fig. 1) and a spectrum taken with Herschel’s SPIRE from 450 GHz to 1.400 GHz. Credit: ESA/Herschel/PACS, SPIRE/MESS Key Programme Supernova Remnant Team

Crab Nebula compound image: HERSCHEL (PACS @ 70 um & HUBBLE

Science, 13 December 2013 Vol. 342 no. 6164 pp. 1343-1345

ArH+ Motivation:

• Recently: Schilke et al A&A 566 A29 (2014)

• J=1-0 → 617.5 GHz, J=2-1 → 1234.6 GHz. Bad atmospheric transmission. As an alternative, after the end of the Herschel mission, future observations willl have to be carried out in the infrared.


Antecedents

• 36Ar is the most abundant isotope in space.

• On Earth: 40Ar 99.600%, 36Ar 0.337%, 38Ar 0.063%. On Earth 40Ar is produced mainly by β-decay from 40K, and marginally from 40Ca.

• Previous spectroscopic work:

Pu

re r

ota

tio

n

Bowman et al J=1-0 40ArD+, 38ArD+ 36ArD+ Sub-mm JCP 79, 2096 (1983)

Liu, Ho, Oka J=20-25, v=1-4 40ArH+ Diode laser JCP 87, 2442 (1987)

Laughlin et al J=1-0 J=2-1

40ArH+ 40ArD+

FIR PRL 58, 996 (1987)

Brown et al J=2-1 to J=7-6 40ArH+ TuFIR JMS 128, 587 (1988)

Odashima et al J=3-2 to J=15-14 40ArD+ TuFIR JMS 195, 356 (1999)

Vib

rati

on

-ro

tati

on

Brault & Davis v=1-0 to v=5-4 40ArH+ FTS PhyScr 25, 268 (1982)

Johns v=6-5 to v=7-6 v=1-0 to v=3-2

40ArH+, 40ArD+

FTS JMS 106, 124 (1984)

Haese & Oka P(2), P(3) v=1-0 36ArH+, Diode laser Unpublished

Filgueira & Blom R(1)-R(3), P(4) v=1-0 P(3),P(4) v=1-0

36ArH+, 38ArH+

Diode laser JMS 127, 279 (1988)

This work P(6) to R(7) v=1-0 R(0) to R(4) v=1-0

36ArH+, 38ArH+

Difference-frequency

APJL 783, L5 (2014)


Experimental results

S/N ~1100 in R(6) 40ArH+

From the linewidth: Tkin ≈ 380-400 K ≈ Trot

From the ratio of intensities in v=1-0 and v=2-1, Tvib ≈ 580 K

From the transition dipole moment and HW factors, N [40ArH+] ≈ 4×1010 cm-3


Results

38ArH+

Up to 800 avgs

36ArH+

Up to 200 avgs


Results

For 36ArH+,

J(1-0)=617524.4 ± 1.2 MHz (from just our polinomial fit) vs.

J(1-0)=617525.23 ± 0.45 MHz (Cologne database from a global fit to all previously available data)

We have used our new wavenumbers in a similar fit to all available data using a Dunham type expansion:

For ArH+ ( Watson, JMS 80, 411 (1980))


Results

• Full results from the fit are in Cueto et al. Astrophysical Journal Letters 783, L5 (2014)

• 367 experimental weighted data.

• Different versions of the reduced mass give significantly different quality fits:

• We have used ② as the charge modified reduced mass; i.e. all the charge is concentrated in the Ar nucleus.

①σw = 0.82 ② σw = 0.72 ③σw = 0.76 ④σw = 0.72


Ion kinetics in Ar/H2 plasmas

sources: sinks:

Ar+ + H2 → ArH+ + H ArH+ + H2 → Ar + H3+

Ar + H3+ → ArH+ + H2 ArH+ + wall→ Ar + H

Ar + e- → Ar + + 2e-

Similarly to the diffuse interstellar medium, too much H2 destroys ArH+

H2 is ejected from the cathode when the discharge is on. (0.002 mbar H2 from 0.4 mbar Ar)

We have studied hollow cathode discharges at 2 different pressures and with H2 fractions from 0 to 100 %

0.08 mbar


Summary

• We have provided direct wavenumbers measurement of rather high accuracy for 19 lines of 36ArH+ (14 lines) and 38ArH+ (2 lines). Of those, only 8 had been reported before, and with much less accuracy.

• These new wavenumbers have improved a new Dunham-type fit to all available published data (U10 , U12 and Δ(Ar)10 uncertainties decrease by factors of 1.8, 1.4 and 5.6 respectively)

• ArH+ should be detectable in dark clouds at T<100 K against bright IR sources.

• Or in emission if Tkin > 1000 K

• Studies of the ion chemistry of Ar/H2 cold plasmas evince that the ArH+ concentration dependes markedly on the electron temperature and the degree of excitation of H3

+


¿Cómo es el trabajo en el «laboratorio de infrarrojo por diferencia de frecuencias»? No se trata de instrumentos comerciales. Aprenderéis: óptica, (lineal y no lineal) electrónica, fontanería, bricolage y McGiverismo en general, algo de astrofísica, (o mucho gracias a nuestros colegas), y incluso Física y Espectroscopía Molecular!


josé luis doménech molecular physics department instituto ...€¦ · josé luis doménech...

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