Potassium and ammonium hydrogen phthalates ... - Wiley Online Library

4

0.6362

KHC6H4(COO)2

0.5632

3

2

1

0

0.7909

Abstract: Raman-induced many-phonons Stokes and antiStokes generation in a orthorhombic crystals KHC6 H4 (COO)2 and (NH4 )HC6 H4 (COO)2 under picosecond pumping has been observed. All recorded nonlinear lasing lines in the visible and near-IR regions are identified and attributed to the χ(3) promoting vibration modes of these orthorhombic phthalates.

0.53207*

Laser Phys. Lett. 6, No. 7, 544–551 (2009) / DOI 10.1002/lapl.200910020

SRS intensity, arb. units

544

ωSRS1 ≈ 1040 cm-1

0.4

0.5

ωSRS2 ≈ 3075 cm-1 0.6

0.7

0.8

0.9

Wavelength, μm

Room-temperature SRS and RFWM spectrum of an orthorhombic KHC6 H4 (COO)2 single crystal c 2009 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA

Potassium and ammonium hydrogen phthalates KHC6H4(COO)2 and (NH4)HC6H4(COO)2 – new organic crystals for Raman laser converters with large frequency shift A.A. Kaminskii, 1,∗ S.N. Bagayev, 2 V.V. Dolbinina, 1 A.E. Voloshin, 1 H. Rhee, 3 H.J. Eichler, 3 and J. Hanuza 4,5 1

Institute of Crystallography, Russian Academy of Sciences, Moscow 119333, Russia Institute of Laser Physics, Russian Academy of Sciences, Novosibirsk 630090, Russia 3 Institute of Optics and Atomic Physics, Technical University of Berlin, 10623 Berlin, Germany 4 Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 50950 Wroclaw, Poland 5 Department of Bioorganic Chemistry, Institute of Chemistry and Food Technology, Wroclaw University of Economics, 50345 Wroclaw, Poland 2

Received: 20 February 2009, Accepted: 23 February 2009 Published online: 5 March 2009

Key words: Raman crystal; KHC6 H4 (COO)2 crystal; (NH4 )HC6 H4 (COO)2 crystal; potassium hydrogen phthalate; ammonium hydrogen phthalate; many-phonon stimulated Raman scattering; SRS; cross-cascaded χ(3) ↔ χ(3) interaction PACS: 42.55.Rz, 42.65.Dr, 42.65.Ky, 78.30.Jw

1. Introduction The use of stimulated Raman scattering (SRS) in crystals to shift the wavelength of laser radiation is becoming more widespread in modern laser physics and nonlinear optics. During the last two decades, this growth in activity has been made possible by the discovery and development of a long list of SRS-active crystalline materials (see, e.g. [1]). ∗

Among them particularly interesting are the heterodesmic organic crystals with predominantly ionic bonds which offer a large Raman frequency shifts (in many examples more than 3000 cm−1 ) and adequate physical properties (see Table 1). Therefore, for our present SRS-investigation we selected two orthorhombic phthalates KHC6 H4 (COO)2 and (NH4 )HC6 H4 (COO)2 , which fall in this category of or-

Corresponding author: e-mail: [email protected] c 2009 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA

Laser Phys. Lett. 6, No. 7 (2009)

545

Crystal

Space group

Nonlinearity SRS-promoting vibration mode at 300 K, cm−1

LiHCOO·H2 O

9 C2v − P na21

χ(2) +χ(3)

LiNH2 C6 H4 SO3 ·H2 O KHC6 H4 (COO)2 c)

5 C2h − P 21 /b 5 C2v − P 21 ab [4]

χ(3) χ(2) +χ(3)

(NH4 )C6 H4 (COO)2

15 D2h − P cab [5] e) χ(2) +χ(3)

[C(NH2 )3 ]2 Zr[N(CH2 COO)3 ]2 ·H2 O D25 − C2221 (GuZN-III) N(CH2 CH2 NH3 )3 Br3 (“tren”·3Br) T 4 − P 21 3 15 α–Ca(HCOO)2 D2h − P bca D24 − P 21 21 21

Y(HCOO)3 ·2H2 O

a) b) c) d) e) f) g) h)

Observed manifestations of nonlinear-laser effects SHG a) , SRS

≈1372, (≈163, ≈113, ≈104, ≈82, ≈79, ≈76) b) ≈1075, ≈1128, ≈111 SRS ≈3075, ≈1040, SRS, cr-casc-SRS d) ≈810, ≈83 ≈3120, ≈1055 SRS

Ref.

[2]

χ(2) +χ(3)

≈2940, ≈1008

SHG, SRS

[3] this work this work [6]

χ(2) +χ(3) χ(3)

≈2965 ≈2892, ≈1387, ≈1352, ≈67 ≈2895, ≈1395, ≈1377

SRS SRS

[7] [3]

SHG, SRS, self-FD (SRS) f ) , self-SFG g) , self-SRS (SFG) h)

[8]

χ(2) +χ(3)

SHG, second harmonic generation. Observed at cryogenic temperatures. In scientific literature this crystal is noted also as: C6 H4 (COOH)·(COOK), K(C6 H4 COOH·COO), KHC8 H4 O4 , C8 H5 O4 K, and PAP (potassium hydrogen phthalate), KAP (potassium acid phthalate). Cr-casc-SRS, i.e. cross-cascaded SRS lasing with participation of two different χ(3) -active vibration modes (in this case of ωSRS1 ≈ 1040 cm−1 and ωSRS3 ≈ 83 cm−1 ) in mixing SRS lasing. ˚ The author of [5] used non-standard setting (Hermann-Mauguin symbol) Pcab, which correspond to the lattice constants a = 6.430, b = 10.240, and c = 26.130 A. The standard setting for the space group No. 61 is Pbca. Self-FD (SRS), i.e. SHG from the arising SRS lasing components. Self-SFG, i.e. self-sum-frequency generation of the arising SRS lasing components and pumping radiation. Self-SRS(SFG), i.e. SRS from the SFG arising from pumping radiation and its Stokes generation.

Table 1 Selected nonlinear-laser characteristics of known SRS-active heterodesmic organic crystals with predominantly ionic bonds between structural units

100

KHC6H4(COO)2 nx ny

80

1.75 1.70

60

1.65 1.60

nz

ny nx

40

nz

20

1.55 1.50 1.45

Transmission, %

Refractive index

1.80

0 0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

Wavelength, μm

Figure 1 (online color at www.lphys.org) Room-temperature wavelength dispersion of refractive indices and transmission spectra (of 1-mm plate cut along c-axes) of an orthorhombic KHC6 H4 (COO)2 single crystal

ganic crystals. They are materials, in which van der Waals, ionic and hydrogen bond forces coexist together. It should

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be noted here that these biaxial non-centrosymmetric crystals offer high piezoelectricity, ferroelectricity, pyroelectricity, and birefringency (see, e.g. [9]). They are also wellknown materials in the acoustic physics and engineering, as well as in plasma physics and in soft X-ray spectroscopic instrumentation (see, e.g. [10]).

2. KHC6 H4 (COO)2 and (NH4 )HC6 H4 (COO)2 crystals for SRS experiments Single crystals of KHC6 H4 (COO)2 and (NH4 )HC6 H4 (COO)2 have been grown from aqueous solution using controlled lowering temperature at a growth velocity of about 1 mm/day. From these crystals we prepared samples for SRS measurements in the form of rectangular bars of 8×5×5 mm3 size with larger dimension along crystallographic c-axis. Their faces were polished plane-parallel but not anti-reflected coated. Some known physical properties of orthorhombic KHC6 H4 (COO)2 single crystal is summarized in Table 2. Potassium hydrogen phthalate single crystal exhibit excellent cleavage and the planes are comparable to that

c 2009 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA

546

A.A. Kaminskii, S.N. Bagayev, et al.: Potassium and ammonium hydrogen phthalates

Property Space group [4] a) ˚ Unit cell parameters, A Formula units per unit cell Local symmetry of atoms Density, g cm−3 Method of crystal growth

Vicker’s microhardness, kg mm−2 Linear optical character Nonlinearity Optical transparency range, μm d) Refractive index Birefringence Elastic stiffness constants, cE ij , GPa [14] −10 Elastic compliances, sE m2 N−1 [14] ij , 10

Poisson’s ratios [14] E Thermoelastic constants, TSij , 10−4 K−1 [12] f) Relative dielectric constants, εS ij [12] Piezoelectric coefficients, 10−8 CSGE [12] Laser-induced damage threshold, J cm−2 Extension phonon spectra, cm−1 h) Frequency of SRS-promoting vibration modes, cm−1 a) b) c) d) e) f) g) h)

5 C2v − P 21 ab (No. 29) a = 6.46; b = 9.90; and c = 13.85 [4] b) α = β = γ = 90◦ Z =4 All in C1 position dm ≈ 1.6364 [11] From aqueous solution by the cooling method (see, e.g. [12,13]); slow evaporation of the aqueous solution [14]; floating seed technique [15]; mercury seal technique [16]; SR method [17] ≈ 350 [18] c) Biaxial (ny > nx > nz ) χ(2) +χ(3) ≈ 0.3 – ≈ 1.45 (see Fig. 1) (see Fig. 1 and [19]) e) Δn ≈ 0.179 [20] c11 =20.10; c22 =15.17; c33 =10.61; c44 =7.64; c55 =5.18; c66 =6.94; c12 =11.11; c23 =5.99; c13 =0.23 s11 =0.91; s22 =1.37; s33 =0.63; s44 =1.31; s55 =1.93; s66 =1.44; s12 = −0.75; s23 = −0.41; s13 =0.22 ν12 = +0.82; ν13 = –0.24; ν21 = +0.55; ν23 = +0.30; ν31 = –0.35; ν32 = +0.65 Ts11 = 9.3; Ts22 = 4.47; Ts33 = 9.0; Ts44 = 9.0; Ts55 = 7.0; Ts66 = 9.0; Ts12 = 1.5; Ts23 = 2.0; Ts13 = 5.0 ε11 = 6.0; ε22 = 3.87; ε33 = 4.34 d15 = –21.5; d24 = 12.9; d31 = –46; d32 = 26.5; d33 = 16.5 ≈ 0.68 g) From ≈ 37 to ≈ 3750 ωSRS1 ≈ 1040; ωSRS2 ≈ 3075; ωSRS3 ≈ 83; ωSRS4 ≈ 810

The author [4] in his structure determination of the title crystal used a non-standard setting, which correspond to the Hermann-Mauguin symbol P 21 ab. The standard setting in the space group No. 29 has the symbol P ca21 . In addition, there are four other settings possible: P bc21 , P 21 ca, P c21 b, and P b21 a. ˚ b = 9.609 A, ˚ and c = 13.857 A. ˚ In accordance with recent measurements [21]: a = 6.466 A, Measurements were carried out on the {010} face at indentation load of about 50 g. For 1-mm plate cut along c-axis. KHC6 H4 (COO)2 crystal has large birefringence value (≈ 0.179) that is of order of calcite (CaCO3 ). Measured at f = 106 Hz and electric field E = 10 V cm−1 . Under nanosecond (65 ns) radiation of Q-switched Nd3+ :Y3 Al5 O12 laser at λ ≈ 1.064 μm and repetition rate 10 KHz [22]. From FT-IR spectra [23] and spontaneous Raman scattering spectra [24].

Table 2 Some physical properties of orthorhombic KHC6 H4 (COO)2 single crystal at room temperature (units of all properties given as in original papers)

of crystalline mica (KMg3 Si3 AlO10 F2 ). Unfortunately, KHC6 H4 (COO)2 crystal is much less studied.

3. Many-phonon Stokes and anti-Stokes generation For investigation of single-pass (cavity free) Raman induced χ(3) -nonlinear generation at Stokes and anti-Stokes wavelengths of orthorhombic single crystals KHC6 H4 (COO)2 and (NH4 )HC6 H4 (COO)2 we used a home-made Xe-flashlamp-pumped picosecond


Nd3+ :Y3 Al5 O12 -laser emitting at two fundamental wavelengths at λf 1 = 1.06415 μm (τp1 ≈ 100 ps, 4 F3/2 → 4 I11/2 channel of Nd3+ activator ions) and λf 2 = 0.53207 μm (τp2 ≈ 80 ps, SHG) (see, e.g. [25,26]). The nearly Gaussian profile of its beam was focused with a lens (f = 25 cm) into an about 20-mm long crystalline sample oriented along c-axis, resulting in beam-waist diameter of about 160 μm. The spectral composition of the SRS and Ramaninduced four-wave mixing (RFWM) lasing of title phthalates was studied with a spectrometric multi-channel analyzing system (CSMA) on the base of a scanning grating monochromator (McPherson Model 270) equipped with

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0

ωSRS4 ≈ 810 cm-1

ωSRS3 ≈ 83 cm-1 ωSRS2 ≈ 3075 cm-1

0.5632

0.6362

KHC6H4(COO)2

3

2

1

0

ωSRS1 ≈ 1040 cm-1

0.7909

1.1645

1.1966

0.9736 0.9797

1

0.9358 0.9431 0.9505

0.8713

0.8018

2


4

3

0.7989


4

0.53207*

0.9581 0.9658

KHC6H4(COO)2

547

1.06415*


ωSRS1 ≈ 1040 cm-1

0.4

ωSRS2 ≈ 3075 cm-1

0.5

0.7

0.6

0.8

0.9

Wavelength, μm 0.7

0.8

0.9

1.0

1.1

1.2

Wavelength, μm

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3

1.1987

1

0.8690

2

0.7989


4

Si-CCD Hamamatsu linear sensor (S3923-1024Q with maximum spectral sensitivity around 0.65 μm). Some obtained χ(3) -lasing spectra and results of an identification and attribution of their Stokes and anti-Stokes components to SRS-promoting vibration modes of KHC6 H4 (COO)2 and (NH4 )HC6 H4 (COO)2 crystals are shown in Fig. 2 – Fig. 4 and summarized in Table 3. Unfortunately, at this stage of nonlinear-laser investigations of KHC6 H4 (COO)2 and (NH4 )HC6 H4 (COO)2 phthalates we can not conduct all required measurements with long samples in other crystallographic orientations due to problems with crystal growth conditions and fine polishing of these crystals to get the needed optical quality of their working surfaces. We hope that before long we can get over these temporary difficulties. Nevertheless, for KHC6 H4 (COO)2 crystal we have roughly estiSt1−1 mated the steady-state (ss) Raman gain coefficient gssR for the first Stokes generation at λSt1−1 = 1.1966 μm wavelength, which related to its SRS-promoting mode ωSRS1 ≈ 1040 cm−1 . We can do it, because our excitation condition of the χ(3) -nonlinear generation regime in KHC6 H4 (COO)2 crystal is essentially steady-state: τp T2 = πΔνR ≈ 2.1 ps (here T2 is the phonon dephasing time and ΔνR ≈ 5 cm−1 is the linewidth of Raman shifted line related to ωSRS1 , see also Fig. 5). For

1.06415*

(NH4)HC6H4(COO)2

0.9567

Figure 2 (online color at www.lphys.org) Room-temperature SRS and RFWM spectrum of an orthorhombic KHC6 H4 (COO)2 single crystal recorded in excitation geometry ≈c(aa)≈c under pumping at λf 1 = 1.06415 μm wavelength (marked by an asterisk). Wavelength of all lines given in mm, and their intensity are shown without correction for the spectral sensitivity of used analyzing SCMA system with Si-CCD sensor (see next). Stokes and anti-Stokes lines related to SRS-promoting vibration modes of the crystal ωSRS1 ≈ 1040 cm−1 , ωSRS2 ≈ 3075 cm−1 , ωSRS3 ≈ 83 cm−1 , and ωSRS4 ≈ 810 cm−1 are indicated by the horizontal scale brackets

Figure 3 (online color at www.lphys.org) Room-temperature SRS and RFWM spectrum of an orthorhombic KHC6 H4 (COO)2 single crystal recorded in excitation geometry ≈c(aa)≈c under pumping at λf 2 = 0.53207 μm wavelength. Stokes and antiStokes lines related to SRS-promoting vibration modes of the crystal ωSRS1 ≈ 1040 cm−1 and ωSRS2 ≈ 3075 cm−1 are indicated by the horizontal scale brackets. Other notations are as in Fig. 2

0

ωSRS2 ≈ 3120 cm-1 0.7

0.8

0.9

1.0

ωSRS1 ≈ 1055 cm-1 1.1

1.2

Wavelength, μm

Figure 4 (online color at www.lphys.org) Roomtemperature SRS and RFWM spectrum of an orthorhombic (NH4 )C6 H4 (COO)2 single crystal recorded in excitation geometry ≈c(aa)≈c under pumping at λf 1 = 1.06415 μm wavelength. Stokes and anti-Stokes lines related to SRSpromoting vibration modes of the crystal ωSRS1 ≈ 1055 cm−1 and ωSRS2 ≈ 3120 cm−1 are indicated by the horizontal scale brackets. Other notations are as in Fig. 2


548 Pumping condition

A.A. Kaminskii, S.N. Bagayev, et al.: Potassium and ammonium hydrogen phthalates Stokes and anti-Stokes generation

λf , μm Excitation geometry a) Wavelength, μm b) 5 KHC6 H4 (COO)2 : space group C2v 1.06415 ≈a(cc)≈a 0.8018 (see Fig. 2a) 0.7989 0.8713 0.9358 0.9431 0.9505 0.9581 0.9658 0.9736 0.9797 1.06415 1.1645 1.1966 0.53207 ≈a(cc)≈a 0.53207 (see Fig. 2b) 0.5632 0.6362 0.7909 15 (NH4 )HC6 H4 (COO)2 : space group D2h 1.06415 ≈a(cc)≈a 0.7989 (see Fig. 3) 0.8690 0.9567 1.06415 1.1987 a)

b) c)

Frequency of SRS-promoting vibration mode (cm−1 ) ωSRS1 ωSRS2 ωSRS3 ωSRS4

Line c)

Attribution

ASt3−1 ASt1−2 ASt2−1 ASt3−3 ASt1−1 ASt2−3 ASt1−1 ASt1−3 ASt1−1 ASt1−1 St1−3 ASt1−1 St2−3 ASt1−1 ASt1−4 λf 1 St1−4 St1−1 λf 2 St1−1 St1−2 St2−2

ωf 1 +ωSRS1 ωf 1 +ωSRS2 ωf 1 +2ωSRS1 ωf 1 +ωSRS1 +3ωSRS3 ωf 1 +ωSRS1 +2ωSRS3 ωf 1 +ωSRS1 + ωSRS3 ωf 1 +ωSRS1 ωf 1 +ωSRS1 –ωSRS3 ωf 1 +ωSRS1 –2ωSRS3 ωf 1 +ωSRS4 ωf 1 ωf 1 –ωSRS4 ωf 1 –ωSRS1 ωf 2 ωf 2 –ωSRS1 ωf 2 –ωSRS2 ωf 2 –2ωSRS2

≈ 1040

ASt1−2 ASt2−1 ASt1−1 λf 1 St1−1

ωf 1 +ωSRS2 ωf 1 +2ωSRS1 ωf 1 +ωSRS1 ωf 1 ωf 1 –ωSRS1

≈ 3120 ≈ 1055 ≈ 1055 – – – ≈ 1055

≈ 3075 ≈ 1040 ≈ 1040 ≈ 1040 ≈ 1040 ≈ 1040 ≈ 1040 ≈ 1040 –

≈ 83 ≈ 83 ≈ 83 ≈ 83 ≈ 83 –

–

≈ 1040 – – – ≈ 1040 ≈ 3110 ≈ 3110

≈ 810 – ≈ 810 –

–

Notation is used in analogy to [27]. The characters between parentheses are (from left to right) the polarization of the pumping and of scattered laser radiation, respectively, while the characters to the left and to the right of the parentheses are the pump and the scattered beam direction, respectively. The use of approximate directions is marked by “≈”. Measurement accuracy is ±0.0003 μm. Used notation; for example, the notation of cascaded lasing line St1−3 ASt1−1 of the KHC6 H4 (COO)2 crystal is defined as the first Stokes component (related to the third promoting vibration mode ωSRS3 ≈ 83 cm−1 ) from the first anti-Stokes emission connected with the first promoting vibration mode ωSRS1 ≈ 1040 cm−1 .

Table 3 Room-temperature spectral composition of χ(3) -nonlinear lasing in orthorhombic single crystals KHC6 H4 (COO)2 and (NH4 )HC6 H4 (COO)2 under picosecond Nd3+ :Y3 Al5 O12 -laser pumping at two fundamental wavelengths at λf 1 = 1.06415 μm and λf 2 = 0.53207 μm (SHG)

this aim we applied the sufficiently tested method based St1 thr Ip lSRS ≈ 30 (see, e.g. on the well-known relation gssR [28]) and on comparative measurements of the “threshold” (Ipthr ) pump intensity (for the confidently detectable first-Stokes signal [2,29]) for the lasing wavelength at λSt1−1 = 1.1966 μm of the studied phthalate and the “threshold” pump intensity of a χ(3) -active reference crystal with nearly the same SRS-active length (tetragonal St1 ≥ 3.1 cm GW−1 cotungstate PbWO4 with known gssR efficient at its λSt1 = 1.1770 μm [30]). Our measurement show that the “threshold” intensity of the first Stokes component of PbWO4 is about 8 times less than for KHC6 H4 (COO)2 at λSt1−1 = 1.1966 μm wavelength. This


St1−1 result allows us to conclude that desired value gssR for −1 studied phthalate is not less than 0.4 cm GW .

4. SRS-promoting vibration modes As shown above the χ(3) -lasing spectra of KHC6 H4 (COO)2 crystal consist of several Stokes and anti-Stokes lines related to four SRS-promoting phonons with ωSRS1 ≈ 1040, ωSRS2 ≈ 3075, ωSRS3 ≈ 83, and ωSRS4 ≈ 810 cm−1 . The assignment of these bands to the respective normal modes can be proposed on the literature data. As mentioned before, the potassium hydrogen phthalate is described in the orthorhombic

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549

KHC6H4(COO)2

Intensity, arb. units

4

≈ 3075

≈ 1040

3

≈ 810 ΔνR2 ≈ 6 cm-1 ΔνR1 ≈ 5 cm-1

2

ΔνR4 ≈ 5 cm-1 ≈ 83

1

ΔνR3 ≈ 7 cm-1 0 50

100

150

200

250

500

1000

1500

2000

2500

3000

Raman shift, cm-1

Figure 5 (online color at www.lphys.org) Two fragments of the first-order polarized spontaneous Raman scattering A1 (LO)-spectrum of an orthorhombic KHC6 H4 (COO)2 single crystal recorded in excitation geometry c(aa)c at room temperature (partly from [33]). Raman shifts of several intense lines are given in cm−1 . The inset shows the molecule structure of crystal studied

5 P 21 ab or P ca21 (C2v ) space group (see Table 2 and [4,5]). Its unit cell consists of four C6 H4 (COOH)COO− anionic formula units and four K+ cations. Since all these ions and atoms occupy the sites of the C1 symmetry. Each unit-cell mode splits into four components: 5 unit cell of the A1 +A2 +B1 +B2 . The orthorhombic C2v crystal studied comprises 72 atoms that have 216 zonecenter degrees of freedom described by the irreducible representation: Γ216 = 54A1 +54A2 +54B1 +54B2 . Among them (A1 +B1 +B2 ) representation describes the acoustic modes and (53A1 +54A2 +53B1 + 53B2 ) the optical modes. (5A1 +5B1 +5B2 ) modes are IR active whereas in the Raman spectra all modes are active. For the external modes: 12 librational modes of four hydrogen phthalate units and 21 translatory lattice modes have to be observed in the Raman spectra. Remaining modes describe the internal modes of the hydrogen phthalate vibrations that are coupled with the rotational motions of these units. The 18 atoms of the single KHC6 H4 (COO)2 molecule give rise to 54 vibrations that contain 48 internal vibrations and 6 external ones (translational and rotational). Vibrational spectra of the phthalate ion have been widely studied since this ion is a model example of a small carboxylic ligand and its functional group is commonly found in natural organic matter. Such studies have been performed for all forms of this compound: phthalic acid, the hydrogen phthalate ion and the phthalate ion, both in the gas phase, solution and solid state [31,32]. The polarized Raman spectra of oriented single crystals of potassium, rubidium, and thallium were studied in [33]. Besides, the hydrogen phthalate ion was the subject of numerous studies on the nature of the hydrogen bond formed in this compound [34] and on the complexation of the carboxylate to metals, mineral surfaces and metal ions [35].

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The most useful results that can be applied in the discussion of the IR and spontaneous Raman scattering spectra of these materials as well as their SRS spectra, are those obtained in the normal coordinate analysis of the o-phthalic acid (oPA) [32] and polarized Raman studies of potassium phthalate [33]. On that basis, the lines observed in the SRS spectrum of the KHC6 H4 (COO)2 related to ωSRS2 ≈ 3075 cm−1 corresponds to the stretching ν(CH) mode of the benzene ring. The Stokes and anti-Stokes lasing lines connected with active phonons ωSRS1 ≈ 1040 cm−1 is also associated with the vibration of benzene ring and should be assigned to the in-plane δ(CH) bending mode. In the Ag Raman spectrum of the oPA single crystal this mode is observed at ≈ 1040 cm−1 [32]. The next Stokes and ant-Stokes lines related to ωSRS4 ≈ 810 cm−1 fit well to the Ag band observed in the spontaneous Raman scattering spectrum of the oPA single crystal at ≈ 810 cm−1 [32]. It corresponds to the out-ofplane γ(CH) vibration of the benzene ring. The Stokes and anti-Stokes lasing lines related to ωSRS3 ≈ 83 cm−1 in the spectrum of the KHC6 H4 (COO)2 appears also in the Ag spectrum of oPA crystal as a strong band at ≈ 83 cm−1 . In the A1 (TO) spontaneous Raman scattering spectrum of KHC6 H4 (COO)2 it is observed at ≈ 83 cm−1 [33]. It was assigned to the twisting mode of the COOH group [32] or to the O· · ·O vibrations of the OH· · ·O hydrogen bonds [33].

5. Conclusion We have discovered the χ(3) -nonlinear laser potential of two heterodesmic organic crystals KHC6 H4 (COO)2 and (NH4 )C6 H4 (COO)2 having predominantly ionic bonds


550

A.A. Kaminskii, S.N. Bagayev, et al.: Potassium and ammonium hydrogen phthalates

5 15 and different orthorhombic space groups C2v and D2h , respectively. The preliminary results of investigations of their Stokes and anti-Stokes generation showed that both phthalates studied offer several SRS-promoting crystal vibrations, among them are the modes with a frequency of more than 3075 cm−1 . We have estimated the first Stokes steady-state Raman gain coefficient for the χ(3) -lasing at λSt1−1 = 1.1966 μm wavelength of St1−1 KHC6 H4 (COO)2 crystal to be gssR ≥ 0.4 cm GW−1 . There are reason to believe that these new SRS-active noncentrosymmetric organic crystal are also promising subjects for the study of cross-cascaded χ(3) ↔ χ(3) nonlinear interactions, just as were observed in non-centrosymmetric organic Y(HCOO)3 ·2H2 O (see Table 1) and in several inorganic crystals (see, e.g. Na3 Li(MoO4 )2 ·6H2 O [26], LaBO2 MoO4 [36], and YPO4 [37]). We plane to finish our nonlinear-laser experiments with KHC6 H4 (COO)2 crystals, when they will be grown as longer oriented samples with needed polished endfaces.

Acknowledgements The investigation reported here was performed within the “Joint Open Laboratory for Laser Crystals and Precise Laser Systems”. Russian coauthors wish to acknowledge partial support from the Russian Foundation for Basic Research and the Program of the Presidium of Russian Academy of Sciences “Extreme laser fields and their applications”, as well as the Technical University of Berlin. One of us (A.A.K.) is grateful to the Alexander von Humboldt Foundation for the “Festkörperphysik” research prize, that allowed him to carry out SRS experiments at the Institute of Optics and Atomic Physics of the Technical University of Berlin. Authors acknowledge L. Bohatý and B.N. Mavrin for important advices and helps.

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