Isotope shifts in theA1 Π −X1 Σ+ system of PN molecule

Isotope shifts in the A 1 II - X 1 X; + system o f P N molecule P

Spectroscopy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085, India MS received 2 June 1984; revised 6 August 1984

Abstract. The emission spectrum o f PN molecule was excited in an electrodeless microwave discharge in sealed tubes containing phosphorus, nitrogen and neon. Isotope shifts of several bands of the A 1 1 I - X ty~+ system were studied using nitrogen isotope. Vibrational assignments of several bands were confirmed.

geywords. Isotope shift; band system; vibrational assignments.

PALS No. 33.10 Gx

1. Introduction

An emission band system consisting of several red degraded bands in the 2400-2900 A region was first reported by Curry et al (1933). The bands were excited in a transformer discharge containing phosphorus vapour and nitrogen. Vibrational and rotational analyses of these bands enabled them to establish that the band system involves the transition I I I - l X +. Recently Ghosh et al (1981) obtained this band system by introducing a trace of P C I 3 in the afterglow of active nitrogen produced by a microwave discharge through helium containing a trace of nitrogen. They have carried out the rotational analysis of several bands involving v' = 0-10 to v" = 0-11 transitions and determined the band origins and molecular constants of the upper and lower states.

In order to study the observed perturbations in the A ~ II state in greater detail, high resolution studies of several bands of A I F I - X I T . + system of P~4N and P~SN molecules are in progress. Preliminary investigations of this band system recorded under low dispersion enabled us to carry out the isotope shift studies and confirm the vibrational assignments of observed bands. Details of the isotope shift studies are presented in this paper.

2. Experimental

The emission spectrum of PN was excited in an electrodeless microwave discharge (2450 MHz) in sealed tubes, containing phosphorus, nitrogen and neon. This type of excitation was chosen mainly to use the nitrogen isotope (tSN2). The discharge tubes were made of vicor tubes (10mm diameter and 6cm long) and were connected to vacuum system and continuously evacuated for several hours and simultaneously degassed using a furnace maintained at 600°C. Trace amounts of phosphorus were then distilled into the tube. After admitting a mixture of pure nitrogen and neon at about 665 P--10

666 P Saraswathy and G Krishnamurty

6 torr in the ratio 4: 1, the tube was sealed. To get a reasonably intense P N spectrum, the optimum power of the microwave oscillator was maintained at 60 W. This power was critical since increase of power resulted in intense emission of P2 bands while at lower powers bands due to N 2 were prominent. The spectrum due to P~SN was obtained using 15N 2 gas enriched to 50%. The A I H - X ~ Y - + bands of P14N and p~SN were photographed on 3"4 m Ebert Jarrel-Ash spectrograph using 1200 grooves/mm grating in the first order at a dispersion of 2"3 A/mm and the spectra recorded (Kodak SA-1 emulsion). Iron hollow cathode lines were used as wavelength standards. The band heads were measured on Zeiss Abbe's comparator. The measurements for sharp band heads are accurate to 1 cm-1.

3. Results and discussion

The bands involving A 1 I1 - X 15: + transition of P14N and p1 s N molecules lying in the 2350-2900A region are shown in figure 1. The bands are degraded to red with the R branch forming the head. In addition to the R head, each band consists of a Q head characteristic o f the transition 11-I-1Z. The band system consists o f well-defined sequences. The band heads due to P~4N and P~SN could be sorted out easily.

All the observed R heads of P14N can be represented by v R = 39816.6 + [1102.18(v' + 1/2) - 7 - 2 8 ( F + 1/2)2,]

- [-1336.01 (v" + 1/2) - 6.96(v" + 1/2)2,]

and the Q heads by

v o = 39805"50 + [1103.0(v' + 1/2) - 7.25(v' + I/2) 2 ]

- [1336.9 (v" + 1/2) - 6.88 (v" + 1/2) 2 - 0.007 (v" + 1/2) 3-]

(i)

(2)

where v R and vQ are the frequencies of the R and Q heads respectively. There is a large difference in lhe value of the system origin in the above expressions which is because o f the separation between the band head and the band origin. Molecular constants used in (1) are approximate since they are obtained from the R head data. However this expression is useful to locate the R head positions to an accuracy of _ 1.5 cm -1.

Whereas the constants used in (2) are accurate since they are derived from the Q head data, and compare well with the values reported by Ghosh et al (1981). The wavelength and wavenumbers of the observed band heads for P14N and P~SN are presented in table 1. The Q heads are also included for most of the bands where the measurements could be done with certainty.

The isotope shifts were calculated using (Herzberg 1950), v - v ' = (1 - p ) [to'e (t~' + 1/2)-(Oe'(r" + 1/2),]

t p i ¢ t ! t t

- ( 1 _ p 2 ) [toex~(t + 1/2) 2 -to~x~ (t + 1/2) 2] (3) where p = 0.9768447, w'~ = 1103 cm -1, eo'ex'~ = 7"25 cm -1, to~' = 1336.9 cm -~ and

~ x ~ ' = 6.88 c m - 1.

The calculated and observed isotope shifts are presented in table 1. The agreement between t h e m confirms the vibrational assignments of the bands unambiguously.

mu N pls N

2338.01

J I <

2477.9 A

I| 3-2 p14N3k-2 4-3 i plS N .,, l 3-2 4-3

]-0

2-3 2,540.4

2555.0 A HIk

P0 (~ SYSTEM) 4-4 5-5 0-1 1-2 i i i !

2666.9,& I

I

2696.9,& p14 N ol-2 1- i ols N

2814.3~1 2819.8/~,1 N2 (2nd+ SYSTEM)

I I

..t J !~

668 P Saraswathy and G Krishnamurty

Table 1. Band heads o f A I F I - X t ~ . ÷ system o f PI4N and PlSN and isotope shifts.

t~r--l) #

P t ' N PtSN Isotope shifts (cm- l)

gait vv~ '~ait v v to v ( P I 4 N ) - vl(pl SN)

(A) (cm- l) (A) (can- ' ) Obs Calc

O-0

0-1

0-2

1-0

1-1

1-2

1-3

1-4

2-0

2-1

2-3

2-4

2-5

3-1

3-2

3-4

3-5

3-6

4-2

4-3

4-4 R

Q

R

Q

R

t2

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

2518'14 3 9 6 9 9 " 8 1 2518.02 39701"78 - 1.97

2518"85 3 9 6 8 8 " 7 4 2518.67 39691"49 - 2 . 7 5 -2.71

2604"90 38377'70 2602.76 38409"26 - 31'56

2605"74 38365'42 2603-50 38398"36 - 32.94 - 33"03

2696.86 3 7 0 6 9 " 1 0 2 6 9 2 " 3 7 37130-96 - 6 1 ' 8 6

2697.76 3 7 0 5 6 " 7 5 . . . . 62.74

245-1"02 4 0 7 8 7 " 0 2 2452.42 40763"76 23"26

2451"62 40776"99 - - - - - - 22'16

2533'25 3 9 4 6 4 " 2 8 2 5 3 2 " 6 9 39471"79 - 7.51

. . . . . . 8.09

2620-02 38156-30 2 6 1 7 " 4 8 38193'25 -36-95

2620-81 38144-79 2618-20 38182"76 -37.97 -37"86

2712.00 3 6 8 6 2 " 2 5 2707.27 36927.94 -65.69

2712"94 3 6 8 4 9 " 4 1 2 7 0 8 " 1 9 36914"05 - 64.64 - 66.93 2809"61 35581.68 2802,18 35675"93 - 9 4 . 2 5

. . . . . . 95"36

2388'28 41858"37 2 3 9 1 " 1 3 41808"50 49.87

. . . . . 46"38

2466.18 40536'34 2 4 6 7 " 2 3 40518.95 17.39

2466.71 40527"59 - - - - - - 16"05

2635"41 3 7 9 3 3 " 4 5 2632-60 37973"97 -40"52

2636"06 37924'12 . . . . 42.71

2727.46 36653'33 2722.42 36721"10 -67-77

2728'33 36641.63 2723.09 36712'17 -70.54 -71.15

2825.05 35387"16 - - - - - -

2403.29 41597.00 2 4 0 5 " 6 3 41556"57 40"43

2403"73 41589"39 - - - - - - 39"60

2481"33 40288"82 2482.00 40277"95 10-87

2481.84 40280"58 - - - - - - 9.90

2650.81 37713'04 2 6 4 7 " 5 6 37759"43 -46-39

2651"39 3 7 7 0 4 " 9 2 . . . . 47.60

2742"90 36446-93 2737.29 36521"63 -74.70

2743.65 3 6 4 3 7 " 0 5 . . . . 75.41

2840-54 35194"18 - - - - - -

2841"50 35182'39 . . . .

2418'76 4 1 3 3 0 " 9 8 2420-70 41297'89 33.09

2419'26 41322-43 - - - - - - 32,79

2496"94 40036"83 - - - - - -

2497.46 40028.58 . . . .

2579.46 3 8 7 5 6 " 1 4 2577.92 38779-36 - 2 3 ' 2 2

. . . . . . 24"03

Table I. (Contd.)

pl't N

' ~ a i t Vvac

if-v" (A) (cm- l )

pl s N Isotope shifts (cm- t)

~'air Vvac v (PI4N)- v ~(plsb0

(A) (cm - l ) O b s Calc

R 2666'69 37488.50

4-5 Q - - _ _

R 2758'81 36236.86

4-6 Q 2759.64 36225.92

R 2856-44 34998.35

4-7 Q 2857.43 34986.19

R 2434"39 41065"60

5-3 Q 2434.87 41057'46

R 2595'41 38517.99

5-5 _{Q - - _ _}

R 2774"89 36026.87

5-7 Q 2775-65 36016.99

R 2872'51 34802'51

5-8 Q 2873.47 34790-88

R 2375.65 42080.82

6-3 Q - - _ _

R - - - -

6-8 Q 2792.06 35805.34

R 2888.75 34606.86

6-9 _{Q - - - -}

R 2905-18 34411-25

7-10

R 2921.92 34214.10

8-11

Q - - _

2752-66 36317"70 -8ff84

2753"55 36306"01 -80'09 -79.70

2435"95 41039-29 26'31

- - - - 25-94

2769"32 36099-44 -82"45 -84'03

2785-22 35893.24 -87.90 -88"39

A c k n o w l e d g e m e n t s

T h e a u t h o r s t h a n k D r S L N G K r i s h n a m a c h a r i , f o r his keen interest a n d suggestions.

R e f e r e n c e s

Curry J, Herzberg L and Herzberg G 1933 Z. Phys. 86 348

Ghosh S N, Verma R D and Van der Linde J 1981 Can. J. Phys. 59 1640

Herzberg G 1950 Molecular spectra and molecular structure 1. Spectra of diatomic molecules (Reinhold, New York: Van Nostrand) p. 162