Analysis of Some New Electronic Transitions Observed using Intracavity Laser Spectroscopy - PowerPoint Presentation

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Analysis of Some New Electronic Transitions Observed using Intracavity Laser Spectroscopy (ILS)

Jack C. Harms, Ethan M. Grames, Leah C. O’Brien,* and James. J. O’BrienUniversity of Missouri – St. LouisDepartment of Chemistry and Biochemistry*Southern Illinois University EdwardsvilleDepartment of Chemistry

1Slide2

Electronic Transitions Observed using ILS

Four transitions with strong blue-degraded bandheads were observed in the orange/red region of the visible spectrum in the plasma discharge of a copper hollow cathodeBandhead positions:(a) 16,560 cm-1(b) 16,485 cm-1

(c) 16,027 cm

-1

(d) 15,960 cm-1The observed transitions are not consistent with any transitions reported in Pearse and Gaydon or any known transitions of Cu containing diatomic molecules

2Slide3

Experimental Methods

Absorption spectra were collected for a molecular species produced in the plasma discharge of a Cu hollow cathode1-1.5 torr H2 or D2 as sputter gasRF power supply to cathode: 0.22-0.60 AThe hollow cathode was located within the laser cavity of a dye laser

R6G/DCM Laser Dye

Verdi V10 pump laser operating at 1.50 W

XYZ translation of highly refractive wedge for tuningCathode lengths ranged from 50-150 mm and generation time for experiments 90μsecLaser cavity ~1.1 m long: effective pathlength ~1 kmA spectrum from an external I

2

cell was collected after each plasma spectrum, and the recorded I

2 positions were calibrated in PGOPHER using the reference data of Salami and Ross. The I2 calibrations were then applied to the corresponding plasma spectra. Average deviations in the calibrations were typically less than ±0.002 cm-1.

3Slide4

H

2/D2Instrument Schematic4Slide5

5

1st Pair of Transitions: No Observed D2 ShiftSlide6

6

2nd Pair of Transitions:D2 Shift of ~3 cm-1

Bandheads

for H

2 Transitions: 15,960.1 & 16,027.6 cm-1 Bandheads for D2

Transitions: 15,957.4 & 16,024.7 cm

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Rotational Analysis

Locations of newly observed transitions:(a) 16,560 cm-1(b) 16,485 cm-1(c) 16,027 cm-1(d) 15,960 cm

-1

Four branches have been identified in each transition: 1 P-type, 2 Q-type, and 1 R-type

Combination differences between branches were used to secure a rotational assignment. It has been found through combination differences that transitions (a) & (b) and (c) & (d) share common exited statesThe two sets of transitions do not seem to have any common states, howeverThe branch patterns are consistent with a 2Σ -

2

Π transition, with spin-orbit splitting of the

2Π state (a)-(b) = -71 cm

-1

(c)-(d) = -65 cm

-1

First lines cannot be definitively assigned due to spectral congestion

Transitions were fit using PGOPHER, assuming the

2

Π is inverted and the excited state is

2

Σ

-

because of initial

expectation was

that the molecule is

CuNH

7Slide8

Combination Differences

8

J’

J”

J”

e

e

e

f

f

1/2

3/2

5/2

2

Σ

2

Π

3/2

2

Π

1/2

e

f

e

f

e

f

e

f

e

f

e

f

1/2

3/2

5/2

3/2

5/2

7/2

f

Δ

1

F’

fe

(J’) =

Q

fe

(J’+1) - P

ee

(J’+1) =

R

ff

(J’)

–

Q

ef

(J’)

Δ

1

F”

ef

=

Q

ef

(J”)

–

P

ee

(J”+1) =

R

ff

(J”)

–

Q

fe

(J”+1)

Δ

1

F’

fe

(J’) =

R

ff

(J’)

Ω

=3/2

–

Q

ef

(J’)

Ω

=3/2

=

R

ff

(J’)Ω=1/2 – Qef(J’)Ω=1/2

Qfe(J’+1) - Pee(J’+1) = Rff(J’) – Qef(J’)

Qef(J”) – Pee(J”+1) = Rff(J”) – Qfe(J”+1)

R

ff

(J’)

Ω

=3/2

–

Q

ef

(J’)

Ω

=3/2

=

R

ff

(J’)

Ω

=1/2

–

Q

ef

(J’)

Ω

=1/2

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Molecular Constants

9Molecular Parameters for 2Σ(-)-2Π Transition with Bandheads at 15,960 cm-1

 and 

16,027

 cm-1

Molecular Parameters for

2

Σ

(-)

-

2

Π Transition with

Bandheads

at 16,485 cm

-1

 and 16,560 cm

-1

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Treatment of the Cathode

The observed transitions have been seen under a variety of conditions, all centered around treatment of the cathode surface prior to installationBest results when cathode surface is polished abrasively and inside of cathode is smoothed with metal fileAlso good results when cathode is soaked in ammonia based cleaning solutionTransitions fade after extended plasma dischargeSome success regenerating transitions by running NH3 plasma to “treat” cathode before resuming H2 dischargesAlso some success using small flow of gas from the headspace of a degassed vial of ammonia cleaner

Use of ammonia as a reagent gas results in many

interferent

lines in region of transitions, making data collection with NH3 present impracticalLimited success at enhancing the red-most transitions with the addition of compressed air to the gas mixture10Slide11

Reaction Conditions

Transitions were first observed in Ar discharges when dye laser system was initially configured and vacuum system had a leak rate of 5 mtorr/minThe A-X transition of CuO overlaps transitions and is very strong in Ar discharges

H

2

greatly enhanced the strength of the transitions and suppresses the CuO transitionsOnce the vacuum system was improved, we were unable to reproduce the transition without treatment of the cathode surfaceOn several occasions, the transitions were observed with shockingly strong intensity in discharges initiated just after cathode was installed in vacuum systemTransition not reproduced in spectra collected the next day after vacuum chamber had been thoroughly evacuatedTransitions are strongest when plasma forms within the hollow cathode, but persist for much longer time when plasma is diffuse and not within hollow cathode

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Implications from Experimental Conditions

Surface conditions of the cathode play major role in molecular productionMolecule contains hydrogen because of observed shift in transition energy when deuterium is used as the sputter gasMolecule likely contains oxygen or nitrogen based on enhanced transition intensity with exposure of cathode to atmosphere or aqueous ammonia solutions12Slide13

Interpretation of Rotational Structure

The J dependence of Δ1F combination differences is given by: Δ1F ( J ) = 2B J + 2BThe slope and intercept of Δ1F vs. J will only be equal for either integer

or

half-integer values of J

The half-integer values of J that satisfy this relationship for the identified branches indicate that the molecule has even multiplicityThe observed branch structure fits very well to a 2Σ - 2Π transitionThe 2 “missing” branches are predicted to be weak and the intensity and density of the identified branches could easily mask weak spectral features

The relative simplicity of the spectrum seems to suggest a linear molecule

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Molecular CandidatesInitially considered both CuNH and HCuN as possible sources for the observed transitionsBoth possibilities eliminated through communications with Wenli Zou:CuNH is predicted to be bent in its ground state with A, B, C values of 16.34 cm

-1

, 0.4415 cm

-1, and 0.4299 cm-1Lowest lying linear state is 2Π, but B value is predicted to be 0.4096 cm-1Bond lengths for Cu-N and N-H would have to be 1.95 Å and 0.71 Å to reproduce H

2

/D

2 shift in rotational constantsHCuN is predicted to be linear in its ground state, but the B value for this 4

Σ

-

state is predicted to be 0.4945 cm

-1

Bond lengths would both need to be 1.95 Å in order to reproduce shift in B values

CuH

has B-value of 7.8 cm

-1

, CuH

2

of 1.5 cm

-1

No indication of splitting of lines that would result from the

63

Cu:

65

Cu isotopic abundance ratio of 69%:31%

The molecule likely does not contain copper

If the molecule is not metal containing, it must be composed of several atoms to result in such a small rotational constant

Several possibilities have been considered, but no matches have yet been found

Least unreasonable match to date is HCCO:

Quasilinear

in is ground state:

X

2

A” [

2

Π] Renner-Teller complex with

A

2

A’ state

(B+C)/2 for HCCO and DCCO are 0.3613 and 0.3283 cm

-1

Experimental B values for H

2

and D

2

species: 0.3600 and 0.3262 cm

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Conclusions

Four new electronic transitions have been observed in the orange-red region of the visible spectrum The transitions are identified by strong blue-degraded bandheadsFour rotational branches have been identified in each transition, and they have been analyzed as two 2Σ(-) –

Hund’s

Case (a)

2Π transitionsSpin-Orbit Splitting: -65 and -71 cm-1B”: 0.344480 (27) and 0.350348 (25) cm-1B’: 0.351377 (26) and 0.355255 (25) cm-1

One of the

2

Σ - 2

Π transitions undergoes a shift of -3 cm

-1

when D

2

is used as a sputter gas instead of H

2

The molecule to which these transitions belong has not yet been identified

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Acknowledgements

National Science FoundationUMSL Department of Chemistry and Biochemistry & Center for Nanoscience16