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OBSERVATION OF TWO =0+ EXCITED ELECTRONIC STATES IN JET-COOLED LaH

Suresh YarlagaddaPh.D Student

Homi Bhabha National InstituteBhabha Atomic Research Centre

Mumbai, India

The 69th International Symposium on Molecular Spectroscopy June 16-20, 2014

Introduction

ExperimentalLaser-induced fluorescence spectroscopy of LaH MoleculesResults and discussion

Summary

Outline

2

3

Introduction

Ground state symmetry was not confirmed experimentally

Missing energy linkage between singlet and triplet manifolds

Low-lying electronic states

High-lying excited electronic states

La(a2D,5d16s2 )+H(2S)

???0

4

8

12

16

20

cm-1 x

103

C1

A1

X1+

32

d32, 3

a31, 2

LaH Molecule

*KK das and Balasubramanian , CPL 172, 372 (1990)

Schematic of Experimental Setup

LIF / TOFMS

10-7

mbar

La Metal rod

Pulse ValveAblation source

Delay/Pulse

GeneratorPulse Valve Driver

Nd-YAG Laser

Excimer

Laser

Dye Laser

Boxcar integrat

or Mono-

chromator

Supersonic Molecular Beam

HV Pulse

He + 2% NH3

(250kPa )

Sequence of the various events in the experiment

Pulse Valve

Ablation Laser

Excitation laser

280 ms

25 ms (FJ)

4

LIF

TOF-MS

Ablation Laser

To high-speed pumps

10-5 mbar10-7 mbar

PI Signal

Ion-OpticsLPI

LIF Signal

Mon

o

PZT pulse valve

He +3%NH3

(250kPa)

Electronic transitions of LaH molecules were first observed by A. Bernard

and R. Bacis in visible region 3-3∆, 1 -S 1 P and 1∆- 1 , P Ground state as 3∆

High level ab initio calculations by K. K. Das and K. Balasubramanian predicted 1 S as the ground state with low-lying 3∆ as the first excited state

Latter R. S. Ram and P. F. Bernath observed A1P-X1 S and d3-a3∆ of LaH. They reassigned the 1 -S 1 P and 1∆- 1 P observed by Bernard as 0+- a3∆ and 32-3∆1

Based on abinitio calculation they assigned 1 S as a ground state . In the absence of intercombination transition they could not establish singlet-triplet interval. LaH molecules were formed in the free-jet by reaction of laser produced Lanthanum metal plasma with 3% ammonia seeded in helium gas

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Lanthanum Hydride (LaH) Molecules

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Our previous work on LaH Molecule Experimental confirmation of 1+ symmetry for the ground state

Six new excited electronic states, one with Ω = 0 and five with Ω= 1 observed and rotationally analyzed in the visible region

Observation of inter-combination transition 0+(3-)-X1+ at 15622 cm-1

Established the missing energy linkage between singlet and triplet manifold

Observed new low-lying excited electronic states i.e. 31,2 ,30,1,2 and 12

The 3 state was predicted to be inverted but observed as regular

Vibrational constants of the X 1+ ground state e = 1418.2 (2) cm-1, exe = 15.6 (7) cm-1 were determined

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Observation & Assignments of new = 0+ excited electronic states Rotationally resolved laser-induced fluorescence excitation bands involving two

excited electronic states at 21970.71 and 22100.31 cm-1 are observed

Both the bands are assigned based on observation of number of branches, combination differences for the lower state and radiative lifetimes

R(J-1)-P(J+1) =F”(J-1)-F”(J+1) = 4B”(J+1/2)-8D”(J+1/2)3

=D2F”(J)

21970 and 22100 cm-1 bands originated from n=0 of the X1S+ confirmed by Combination Differences(CD)

J

J-1

J+1

R(J-1)P(J+1)

J

0 20 40 60 80 100 120 140 1603.99

4.00

4.01

4.02

4.03

4.04

4.05

4.06

4.07

Bcal + error Bcal - error

21970 cm-1

22100 cm-1

B"

(cm

-1)

J(J+1)

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Rotationally resolved new = 0+ excited electronic states

Rotational structure of Ω=0+ excitation bands of jet-cooled LaH molecule at 21970 and 22100 cm-1. Lanthanum atomic lines (La I) indicated by *. The simulated spectrum (inverted) was generated with derived molecular constants in Table, a spectral line width of 0.15cm-1 and an estimated rotational temperature of 120K.

21880 21920 21960 22000 22040 22080 22120

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**

*

P Branch R Branch

P Branch

6207 5 4 3 2 1 0 2 5 10

12345679101112

R Branch

*

Wavenumber (cm-1)

Simulated

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Molecular constants for new excited electronic states

3V2

VVV )1(1JJD1JJBTH JJH

The observed rotational lines were fitted in following expressions for the excited states with Ω=0+ for determining molecular constants

Hamiltonian for state with X1S+ and Ω = 0 :

Molecular constants (in cm-1) for the 21970 and 22100 cm-1 bands

Constants Band at 21970 cm-1 Band at 22100 cm-1

Tv 21970.71(2) 22100.31(3)

Bv 3.5065(15) 3.2899(45)

Dv104 -4.44(28) -52.03(163)

Hv105 -3.8(2)

Lv107 1.122(76)

Fitting error(No. lines)

0.03(13) 0.06(21)

(ns) 37.1(38) 39.5(22)

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Dispersed Fluorescence spectra of two = 0+ exited bands

Both the bands are weak in intensity while the monochromator was parked at the excitation laser wavelength

Strong fluorescence to n=1 of the b31 state displaced by about 5066 cm-1 from the laser line

0 1000 2000 3000 4000 5000 6000

A1=0

b3

=1X1 +

=0

b31

=1

b31

=0

a3 1

=1

a3 1

=0

Displacement (cm-1)

Both bands shows transitions to mainly triplet states

Both states having short radiative lifetime 40 ns

The probably assignment is = 0+ in Hund's case (c), as usual for heavy molecules

No theoretical predictions are available for these new electronic states

Electronic states of LaH Molecules

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0

4

8

12

16

20

cm-1 x

103

32

E1D1

(3)

C1

d32, 31

2

A13

0,1,2

a31, 2

X1+

F1

G0+H0+

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Summary

Two new excited electronic states of LaH were identified at T0=21970.7 and 22100.3 cm-1 in rotationally resolved laser-induced fluorescence jet-cooled excitation spectra

The observation of a single P- and R-branch structure in both the bands originating from n= 0 of X1+ground state confirms = 0+ for both the excited states

The molecular constants for both the excited states were determined using Pgopher programme

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Thank You

AcknowledgmentsDr. S.G Nakhate (Ph.D supervisor)

Dr. Soumen Bhattacharyya

Mr. Sheo Mukund

Travel financial assistance by

1. Homi Bhabha National Institute (HBNI)

2. Departments of Science and Technology (DST)

3. Centre for International Co-operation in Science (CICS)