PHOTOTRANSFORMATION OF ORGANIC DYES UNDER POWERFUL LASER EXCITATION.

R. Kuznetsova*, E. Breusova, Svetlichnyi V.

Siberian Physical Technical Institute by Tomsk State University, 1, sq. Novosobornaya,

Tomsk, 634050, Russia

Е-mail: kopylova.@.phys.tsu.ru

The lasers on the base of organic dyes solutions, excited of excimer lasers are the power tunable sources of coherent radiation. The successful application of such lasers is hindered by low efficiency of pump energy transformation and especially stability of the exit lasing characteristics under powerful laser excitation. Therefore there are problems of support of high efficiency and photostability of laser–active media in such conditions. Thus, we investigate the ways of increase the power and resource characteristics of dye lasers, by research of intra- and intermolecular interactions and exciting condition influence on radiating, photochemical and exit parameters of lasers.

The aim of this work is the research of fluorescent properties features of organic molecules depending on their structure, study of phototransformations mechanism of laser media on the base of substitute oxazole dyes under powerful ХеС1 laser excitation, estimation of lasing parameters, photostability and their life time.

R: Н (4РуРО)

R: ОСН3, N(СН3)2 (4РуРООСН3, 4РуРО N(СН3)2)

Figure 1. The molecular structure of researched dyes.

The structure of molecul makes possible participation in intermolecular interactions, the various ionic forms and conformations forming as in the ground, as in the excited states depending on polarity of solutions. It allows to look after influence of intermolecular interactions on the features phototransformation and photoprocesses in molecules under powerful laser excitation.

The stationary spectra were studied on spectrophotometer " Specord M40 " and spectrofluorimeter " Hitachi 850". The lasing spectra on the one pulse were measured with l -meter "Angstrom". The experimental study of spectral, fluorescent and lasing characteristics and photostability for hexan, penthan, ethanol, acetonitril solutions was produced. Our pump source was ELAN (l las=308 nm, t FWHM=15 ns, Eout=40 mJ/pulse) excimer XeCl laser. The transverse pumping variant was applicated.

Table. The spectral-luminescent, lasing characteristics and photostability of researched compounds.

Dye

Solvent

Concentration Mol/l

l abs

nm, sm-1

l fl

nm, sm-1

Stoces shift

sm.-1

Efficiency

%

l gen nm

Life time Р80

J/sm3

j ´ 10-3

Pg

4РуРО

ethanol

ethanol

penthan

5´ 10-4

 

2´ 10-3

 

5´ 10-4

322 (31100)

322 (31100)

308

(32500)

400 (25000)

400 (25000)

6100

 

6100

 

16

 

15

 

3

398

 

398

 

-

16

 

156

 

6

1,8

 

1,6

 

0,7

0,06

 

0,04

 

5,2

4РуРО

N(СН3)2

ethanol

ethanol

hexan

 

 

 

 

acetonitril

5´ 10-4

 

10-3

 

5´ 10-4

 

10-3

 

10-3

382 (26200)

382 (26200)

365 (27400)

365 (27400)

370 (27000)

560 (17800)

560 (17800)

417 (24000)

417 (24000)

550 (18200)

8400

 

8400

 

3419

 

 

3419

8800

9

 

14,4

 

4,5

 

6,5

 

19,5

599

 

599

 

418

 

418

 

589

6,8

 

22,2

 

1,4

 

5,2

 

3,9

2

 

1,1

 

3,5

1,2

 

 

2

0,07

 

0,03

 

 

 

0,42

 

 

0,7

4РуРО

ОСН3

ethanol

hexsan

10-3

 

10-3

340 (29400)

338 (29600)

438 (22800)

375 (26700)

6600

 

3000

6,2

 

2,5

448

 

378

12

 

1,8

2,9

 

2,3

0,07

 

1,8

Two geometry of pumping beam was used. In the first variant the pumping beam is focused in a rectangle with the size 0,08´ 0,8 sm., and density of pump reaches » 50 MW/sm2, that corresponds to conditions of optimal lasing (efficiency of 4РуРОN(СН3)2 in ethanol » 14%). In the second variant the pumping beam is a cylinder with diameter 0,12 sm. and height 0,1 sm. that give increase density of pump up to » 200 MW/sm2, but the lasing of 4РуРОN(СН3)2 at the investigated concentration is absent. The technique of definition of the photochemical characteristics (j - quantum yield phototransformation) and lasing photostability (R80 is life time of active media) is given in work [1].

The spectral-luminescent and lasing characteristics, photostability of researched compounds with the nonselective cavity are listed in Table. 4PyPO is the base molecule for researched pyridylphenyloxazoles, and maximum of laser on 398 nm in ethanol (Table). The lasing efficiency ОСН3, N(CH3)2 - substituted of 4PyPO no was better, but region of radiation larger (Table). We experimental found [2] that the 4PyPON(CH3)2 in the polar solvents have the anomal properties: large stokes shift (Table) and low quantum yield fluorescence (g =0,3 for D n =120 nm). This molecule formed TICT – conformation in the excited states in the polar solvents. Also there are the features in formation dicathion form of this molecule: the second proton interaction to atom of nitrogen dimethilaminogroup as in ground, as in the excited S1 states and efficiency of forming dichation in excited state rises compare ground state. For molecules 4РуРО and 4РуРООСН3 the second proton interaction to atom nitrogen of oxazole cycle. The cathion of 4PyPO, 4PyPOОСН3, 4PyPON(CH3)2 is formed by interaction of proton from solution to pyridyl nitrogen.

The lasing characteristics of 4РуРОN(CH3)2 in the polar solvents were better, them in nonpolar solution (Table). This can be explain by facility of inverse population creation because TICT photoconformation in the ground state is absent.

Two photoproducts, which have absorption in wave of pump radiation (Pp), and in the field of lasing (Рg) are formed. Their structure has not dependence on energy density pulse and polarity of solvent. The photoproduct Рg was formed by interaction of electron-aceptor intermediate particles and the proton-aceptor centres of excited dye. It has spectral characteristics the same the photocathion for 4РуРО and 4РуРООСН3, and photodicathion for 4РуРОN(CH3)2.

The photoproduct Рg of 4РуРО and 4РуРООСН3 radiates in more long-wave, in comparison with initial dye. The nitrogen atoms of pyridyl cycle and dimethilaminogroup are reaction centers of 4РуРОN(CH3)2 forming of Рg photoproduct. Such photoproduct absorbs in more short-wave in comparison with lasing region. It is explained higher life time of 4РуРОN(CH3)2 in polar media in comparison with the nonpolar solvents (Table).

The quantum yield of phototransformations is higher in the nonpolar media (hexan, penthan), than in ethanol (Table). The yield of Pp photoproduct determines molecular photostability of substituted pyridylphenyloxazoles [1]. The Pp photoproduct is a result of break oxazole cycle in the exited state with participation of solvents proton by mechanism predissociation, and the photolis products participate in formation of the Рg photoproduct. The efficiency of photolis in nonpolar solutions is reduced, because there are no free protons (Table).

It is marked, that the phototransformations mechanism of laser-active media on base of 4РуРОN(СН3)2 does not depend from variants of excitation because photoproducts are the same. However, the efficiency of phototransformation is different in the variant 1 and in the variant 2. In the first variant the quantum yield of phototransformations is reduced from 1,7´ 10-3 up to 1´ 10-3 with growth of density pump up to 50 MW/sm2. In the second variant of excitation the photostability is considerably improved: j =3´ 10-4.

The dependencies of the transparency on the power density pump are studied. The transparency decreases from 47 % at linear absorption up to 16 % when power density pump increase up to 250 MW/sm2. It seems the existence of the induced absorption from excited S1 state during excitation pulse reduces the population of photoreactive state and rate of phototransformations.

Thus, the media on the base of dyes forming ТIСТ conformation can produce laser radiation. The features in formation cathion forms and photoproducts 4РуРОN(СН3)2 are explained anomal properties of ТIСТ conformations. The irreversible photoproducts of substituted оxazole are formed by interaction of the protonoacceptor centre of excited molecule and protonodonor or electronaceptor intermediate. Efficiency of phototransformation is depending on the excitation and radiation condition.

Literature.

1. Kuznetsova R., Kopylova Т., Degt’arenko К., Mayer G., Sergeev А., Nesterenko S., Afanas’ev S., Vereskun V. // Quantum Electronics. 26(9) 773-776 (1996).

2. Breusova H., Kuznetsova R., Mayer G.// Atmos. Oceanic Opt. 10(11) 858-859 (1993).