Laboratory Astrophysics: Gas Phase Experiments

Outline

- a few words about Vacuum and Number Density - Modern Vacuum Systems

Vacuum in Space and on Earth

Gas Phase Spectroscopy

- The difficulties with spectroscopy of interstellar molecules - Beer’s law and ways to reach high sensitivities - Gas phase spectroscopy with Ions and Radicals

Reaction Rates and Dynamics - Laboratory Experiments of Neutral-Neutral Collisions - Laboratory experiments of Ion-Neutral Collisions - Electron Recombination Measurements

Gas Phase Spectroscopy: Basics

Beer’s law: 𝑇 = 𝐼

𝐼0

= 𝑒−𝛼 𝐿

transmission incoming intensity

transmitted intensity

absorption coefficient

path length

incoming light

transmitted light

gas cell

Light source

Monochromator (wavelength filter)

detector

Simple Experiment

Challenges in Laboratory Spectroscopy of Interstellar Molecules

- Many interstellar molecules are radicals or molecular ions, they are difficult to produce in large quantities under terrestrial conditions. - The enormous pahtlength that is typically sampled in interstellar observations can not be reproduced in the laboratory - Interstellar clouds have very low temperatures, therefore absorption spectra involve only lines originating from the lowest levels of the molecules. Laboratory preparation of gas samples at 10-100 K can be difficult. Measurements at higher temperatures are difficult to analyze because lines may overlap: spectral congestion. - To determine accurate line frequencies, narrow linewidths are essential, the Doppler width due to the molecular motion at high temperatures can be a problem.

Ways to beat Beer’s law

1) try to get the sample density as high as possible (difficult with ions and radicals)

2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy

3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion

Discharge for Spectroscopy of Molecular Ions (H3+, CH5

+, etc …)

Group of Takeshi Oka, University of Chicago

Discharge for Spectroscopy of Molecular Ions (H3+, CH5

+, etc …)

Group of Takeshi Oka, University of Chicago

Discharge length: 1m Ion density: 1013 cm-3

Neutral/ion ratio 106

Ways to beat Beer’s law

1) try to get the sample density as high as possible (difficult with ions and radicals)

2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy

3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion

Discharge for Spectroscopy of Molecular Ions (H3+, CH5

+, etc …)

Thesis B.J. McCall, Group of Takeshi Oka, University of Chicago

Cavity Ringdown Spectroscopy

without gas in the cell

with gas in the cell

With absorbant: τ = 𝑡𝑟

2[ 1 − 𝑅 + α 𝐿] cavity length

absorption coefficient

τ = 𝑡𝑟

2[ 1 − 𝑅 ]

round trip time of the light in th cavity

mirror reflectivity (0.9999)

decay constant (33 μs)

𝐼 𝑡 = 𝐼0𝑒( −𝑡τ ) Empty cavity:

Ways to beat Beer’s law

1) try to get the sample density as high as possible (difficult with ions and radicals)

2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy

3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion

E1

E2

hν

B1

2 ρ

B2

1 ρ

A2

1

ρ: energy density of the radiation field

Absorption Spontaneous

emission

Induced emission

Einstein Coefficients and Light Amplification Condition

𝐵12= 𝑔2

𝑔1𝐵21

𝐴21= 8𝜋ℎν3

𝑐3 𝐵21

absorption Induced emission

Spontaneous emission

R12 = N1 B12 R21 = N2 B21

Rate of absorption Rate of induced emission

For laser/maser action: R21 > R12

Since B21 = B12

Assume (g1=g2)

N2 > N1

Population inversion is a condition for light amplification through stimulated emission

Laser Principle (3-Level)

Exte

rnal

exc

itat

ion

Spontaneous emission

Short lived

Long lived N2

N1

Stimulated Emission Laser action

Population Inversion N2 > N1

- Stimulated emission (coherent) has to compete with spontaneous emission (incoherent). - Since 𝐴21~ ν3 , light amplification much easier to realize at longer wave length (Masers were realized before lasers).

𝐴21= 8𝜋ℎν3

𝑐3 𝐵21

Laser Principle (4-Level)

Exte

rnal

exc

itat

ion

Spontaneous emission

Short lived

Long lived N2

N1

Stimulated Emission Laser action

Population Inversion N2 > N1

Spontaneous emission

Short lived

Laser Scheme

External Excitation Source

mirror

semi-transparent mirror

Laser medium

Coherent Light beam

Example: Ruby Laser

Tunable Light Sources: Dye Laser, Titanium:Sapphire Laser

Dye Laser Titanium:Sapphire Laser

Output power up to 2W

Dye Laser Coverage

Laser Diode Coverage

But: Laser diodes typically have a very narrow tuning range (30nm), one has to buy a new one if one goes beyond that range

Modern Tunable Laser Systems in the Infrared

OPO - Optic Parametric Oscillator System (2-5 μm)

Splits one pump photon into 2!

Ways to beat Beer’s law

1) try to get the sample density as high as possible (difficult with ions and radicals)

2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy

3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion

Discharge for Spectroscopy of Molecular Ions (H3+, CH5

+, etc …)

Group of Takeshi Oka, University of Chicago

Liqiud nitrogen cooled discharge (77K)

The Benefit of Low Temperatures

H3+ Spectrum

2000-3500 cm-1 Tvib = Trot = 3000 K

H3+ Spectrum

2000-3500 cm-1 Tvib = Trot = 100 K

Supersonic Expansion

- High pressure gas is relaxed through a small nozzle - Molecular velocities exceed the local speed of sound - Heat energy is converted into translational energy in collisions - Rotational temperatures between 0.5 – 30 K can be reached

Cavity Ringdown Spectroscopy with Supersonic Expansion

Ways to beat Beer’s law

1) try to get the sample density as high as possible (difficult with ions and radicals)

2) Create a long pathlength through the sample multipath setups and cavity-enhanced spectroscopy

3) Use strong light sources and sensitive detection techniques 4) Try to get the sample as cold as possible avoid spectral congestion

Adler et al. , Annual Review of Analytical Chemsitry 3, 175 (2010)

Precision Spectroscopy in Discharges

From webpage: Menlo Systems

Revolution in Precision: Frequency Combs

Source: NIST webpage

Revolution in Precision: Frequency Combs

Beat frequency in the microwave domain, Can be counted with standard electronics

NICE OHMS Ion Beam Spectroscopy

Mills et al., JCP 135, 224201 (2011)

PAUSE

Vacuum in Space and on Earth

Ideal Gas law: 𝑝𝑉 = 𝑛𝑅𝑇 𝑛 = 𝑝𝑉

𝑅𝑇

T = 273.15 K V = 1 cm3 = 1 x 10-6 m3

R = 8.314472 J mol-1 K-1

1 mol = 6.02 x 1023

Environment Number density [cm-3] Pressure [mbar]

Earth’s atmosphere 2.7 x 1019 1013.25

Dense interstellar clouds 1 x 104 3.8 x 10-13

Diffuse interstellar clouds 100 3.8 x 10-15

Average Iinterstellar medium 1 3.8 x 10-17

(Very) Approximate density of the Universe

1 x 10-6 3.8 x 10-23

Best laboratory pressure

Collision Time Scales

Collision rate = Rate coefficient x number density

𝑅 = 𝛼 𝑛

Neutral-neutral collision in Earth’s atmosphere: α = 10-13 cm3 s-1 , n = 3 x 1019 cm-3

Example 1)

R = 3 x 106 s-1

(3 million collisions per second)

Example 2)

ion-neutral collision in diffuse ISM: α = 10-9 cm3 s-1 , n = 100 cm-3

R = 1 x 10-7 s-1

(3 collisions per year)

Different Vacuum Regimes

Parameter to judge the pressure regime (or quality) of a vacuum system:

𝐾𝑛 = λ

𝐿 Knudsen number:

λ : mean free path 𝐿 : characteristic dimension

𝐾𝑛 > 0.5 molecular regime (statistical mechanics)

0.5 >𝐾𝑛 > 0.01 transition regime

𝐾𝑛 < 0.01 viscose flow regime (fluid mechanics)

Is Interstellar Space a Good Vacuum?

Consider a diffuse molecular cloud: - The number density is very low for terrestrial standards (100 cm-3)

- On the other hand: the extension of the “vacuum system” is huge!

(on the order of parsec). Therefore the dynamics of a particle is largely determined by collisions with other particles in the system.

The mean free path may be on the order of millions of kilometers, but not as large as the cloud dimensions

𝑲𝒏 ≪ 𝟎. 𝟎𝟏 Interstellar Space is a “Bad Vacuum” !

The small Knudsen number means that Interstellar Clouds are dynamical environments, with paticles undergoing many collisions before they leave the system. Otherwise the dynamics in the ISM would be very boring, and no reactions would occur.

Vacuum generation: Now and Then

Magdeburg hemispheres (Otto von Guericke 1656)

Equipment on exhibition at the Deutsche Museum (Munich)

Pumping techniques (somewhat obsolete)

Oil diffusion Pump Ultimate pressure 10-9 mbar

Rotary Vane Pump Ultimate pressure: 10-3 mbar)

Modern Pump Systems

Scroll pump (rough vacuum 10-2 mbar)

Turbo-Molecular Pump (ultra-high vacuum 10-9 mbar or better)

The Test Storage Ring at the Max-Planck Institut, Heidelberg

55.4 m circumference P ≈ 5 x 10-11 mbar

Ion Pump / Titanium Sublimation Pump

The Cryogenic Storage Ring CSR at MPIK

Cryogenic Storage Ring CSR at MPIK, Heidelberg An Ultracold Storage Ring for Molecular Ions Temperature < 10 K Pressure < 10-13 mbar Electrostatic deflection

no mass limit ideal for molecular ions

The CSR Cryostat Design

Neutral-Neutral Experiments: Crossed Beams

Group of Ralf I. Kaiser Department of Chemistry, University of Hawai’i at Manoa

(Y. T. Lee: Nobel prize in Chemistry 1986)

Examples: HCCCCH on CN (molecular clouds, Titan) B on C2H2, C2H4, CH3CCH, etc …

The Cresu Technique

I. Smith, Annu. Rev. Astron. Astrophys. 49, 29 (2011)

H2+

H3+

CH+

CH2+

CH3+

CH5+

CH4

C2H3+

C2H2

C3H+

C3H3+

C4H2+

C4H3+

C6H5+

C6H7+ C6H6

H2

H2

H2

H2

H2

C

e

C+

e

C+

C

H

C2H2

H2 e

OH+ H2O+

H3O+ H2O

OH e

O

H2

H2

HCO+ CO

HCN CH3NH2

CH3CN

C2H5CN

CH CH2CO

CH3OH

CH3OCH3

CH3+

C2H5+ e

C2H4

e

C3H2 e

C3H

e

C2H

McCall, PhD thesis, Chicago 2001

Ion – Neutral Reactions: Engines of Interstellar Chemistry

Molecular abundances largely determined by the competition between ion-neutral reactions and dissociative electron recombination.

Ion – Neutral Reactions: Engines of Interstellar Chemistry

I. Smith, Annu. Rev. Astron. Astrophys. 49, 29 (2011)

The Flowing Afterglow / Selected Ion Flow Tube (SIFT)Technique

SIFT Instrument In Boulder/Colorado Snow & Bierbaum, Annu. Rev. Anal. Chem. 1, 229 (2008)

5 cm

Cold Ion-Neutral reactions: Radiofrequency Ion Traps

Gerlich, Physica Scripta, T59, 256, (1995)

Quadrupole 22-Pole

22-Pole field Geometry

-1

-0.5

0

0.5

1

-1

-0.5

0

0.5

1

0

0.5

1

1.5

2

-1

-0.5

0

0.5

1

-1

-0.5

0

0.5

1

-1

-0.5

0

0.5

1

-1

-0.5

0

0.5

1

0

0.5

1

1.5

2

-1

-0.5

0

0.5

1

-1

-0.5

0

0.5

1

Example: CH4+ + H2 CH5

+ + H measured in a 22-pole ion trap at 15K

Asvany, Chem. Phys. 298, 97 (2004)

Merged Beams

Bruhns et al, Rev. Sci. Instrum. 81, 013112 (2010)

Interaction region H2 molecule formation

e- ejected

H2- complex

H2 molecule

H- + H H2 + e-

H2+ detector

Laser Helium gas cell

H- ion source 10 kV

Detection region H2 stripping in helium

H2+ detection by energy analyzers

H2 + He H2+ + [He, e-]

e- ejected

H2+ ion

1 m Beam profile monitors

Photodetachment region Partial neutralization of the H- beam

inside a drift tube at variable voltages -Uf

H- + νIR H + e-

-Uf

Example: H- + H H2+e associative detachment

H+ detector

H- dump

cw Nd: YAG Laser cavity / 1-2 kW

H0 dump

demerge magnet

1 m 0

Merged Beams: Neutral-Ion Collisions

spherical deflector

H- beam 9 keV

Ion beam from ECR (20-270 keV)

Neutral H beam

(Oak Ridge National Lab, TN, USA / courtesy C.C. Havener)

Charged products detctor

An Advanced Ion-Atom Collision Setup at CSR

neutral beam (C, 40 keV)

single particle detector

reaction product (CH+, 43.3 keV) stored molecular

ion beam (H3+ , 10 keV)

AIACS at CSR

Example: H3

+ + C CH+ + H2

Ion-Atom Reactions: State-of-the-Art

Flowing Afterglow/ Selected Ion Flow Tube

AIACS

Atoms studied

Ions studied

Absolute measurements

Temperature 300 K

H, O, N

H3+, O2

+, C6H6+, C2

-, … ions stable under plasma conditions

Often only relative calibration possible

40-40000 K

H, D, C, O

H2+, HD+, H3

+, O2+, C6H6

+, H-, HC2

-, C2-, … ,C10

-, ions stable under interstellar conditions

Electron Recombination: Flowing Afterglow

Flowing Afterglow Setup, Charles University Prague

M. Tichy, AIP Conf. Proc. Ser. 669, 60 (2003)

Electron Recombination: Stationary Afterglow

Advanced Stationary Afterglow (AISA), Charles University Prague

J. Glosik, J. Phys: Conf. Ser. 4, 104 (2005)

Electron-Ion Dissociative Recombination Measurements in Heavy-Ion Storage Rings

• radiative relaxation (rotations, vibrations) • direct measurement

• 100% detection efficiency

• high resolution

Advantages

Storage Rings: ASTRID (Aarhus, Denmark) CRYRING (Stockholm, Sweden) TARN II (Tokio, Japan) TSR (Heidelberg, Germany)

no longer active

Examples: H2

+, H3+, DCO+, HCO+, ….

Example: Storage Ring Rate Coefficient Measurement

CRYRING 2003 (n-H2)

TSR 2009 (n-H2:Ar 1:5)

H3+ + e- H2 + H

H + H + H

Kreckel et al., PRA 82, 042715 (2010)

Literature

General: Ian W.M. Smith: “Laboratory Astrochemistry: Gas-Phase Processes” Annu. Rev. Astron. Astrophys. 49, 29-66 (2011)

Electron Recombination:

Mats Larsson and Ann E. Orel “Dissociative Recombination of Molecular Ions” Cambridge University Press. 2008

Ion-Atom Collisions:

T. Snow & V. Bierbaum “Ion Chemistry in the Interstella Medium” Annu. Rev. Anal. Chem. 1, 229 (20-8)