The used chemicals imported from MERCK. The Cr(CO)₄(PPh ₂ was prepared in the following methods: 0.65-g triphenylphosphine PPh₃, 0.1-g NaBH₄, and 0.25-g Cr(CO)₆ were placed in 0.25-L flame dried flask, then about 0.015 L of pure ethanol oxygen free was poured over the flask. The mixture in the vessel was connected to evacuating system with nitrogen gas (purity 99.999%) bubbler. The flask was evacuated until ethanol boiling, and then the N₂ gas was inserted into the flask, this operation was repeated five times to remove any remaining oxygen. Then, the mixture was warm up under refluxed at silicon heating oil bath temperature (85 C) under nitrogen for 3 h, using continuous magnetic stirring, while flushing the flask by a nitrogen flow gas. The final product was filtered hot. The final product (as a solid) was washed 3 times with 0.005 L of distilled cold water and then 3 times with 0.003 L of cold pure ethanol. The final obtained compound looks yellow powder, kept to dray in darkness at room temperature for 24 h.

The obtained material (as shown in Fig. 11) was analyzed by different techniques, such as: Fourier transform infrared (FTIR), nuclear magnetic resonance ³¹P NMR, and ultraviolet-visible (UV-Vis). The FTIR spectrum was recorded as KBr disc in 400 cm –4000 cm using Thermo FTIR 6700 instrument with a resolution at 4 cm and 64 scan. ³¹P NMR spectra were acquired by a Bruker Bio spin 400 spectrometer using CHCl₃ as solvent. The UV-vis absorption spectrum was recorded in the wavelength range 190 nm–1100 nm using UV-1601 PC Shimadzo Spectrophotometer. The NMR chemical shift for the raw triphenyl phosphine materials shows that, the ³¹P{¹H} (200 MHz) δ ppm (s,1P) center at ppm, while the NMR chemical shift for the Cr(CO)₄(PPh ₂ shows that, the ³¹P{¹H} (200 MHz) δ ppm (s,1P) centers at 74.04 ppm. The melting point of the complex is 295 C.

Fig. 1.

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	Fig. 1. (color online) Chromium-triphenyl phosphine cis-complex structure.

IR (KBr, ν, cm ): 1877, 1909, and 1918 (C=O), 1089 (P–Ar), 970, 1280, 1433 (P–C), above 3000 cm (C–H sym., and asym. stretch); The UV-vis spectrum of Cr(CO)₄(PPh ₂ shows two maximum absorption peaks around 262 nm and 335 nm.

2.2. -scan measurements

TEM₀₀ Gaussian beam from continuous-wave (CW) diode laser at 26 mW ( nm) was used as the light source. A 10-cm converging lens was used to focus the laser beam. The radius of the beam ω₀ and diffraction length was calculated to be 33.5 m and 5.5 mm, respectively. The radius of the aperture ( ) was 0.4 mm and the beam waist ( ) on the aperture was 1 mm. The sample cell is 2 mm which was fixed on a computer-controlled translation stage, and it was precisely moved through the focal region of the beam over a length of 6 cm. At the same time, the reference beam energy, and transmitted beam energy were measured using an energy meter (PM300E). Consequently, the transmitted signals were acquired, stored, and later on processed by a computer. The prepared solution was made by accurately weighed amount of the tetracarbonyl-chromium-triphenyl phosphine compounds and dissolved in dichloromethane to obtain samples with a concentration of 10⁻³ M. The experimental set-up was described in full details in Ref. [18].

The Cr(CO)₄(PPh ₂ compound was isolated as yellow powder and characterized using the flowing techniques: ³¹P{¹H} NMR, FTIR, and UV-Vis spectroscopy. The ³¹P{¹H}NMR spectrum of the triphenyl phosphine raw materials gave one singlet peak for P centered at ppm, whereas the ³¹P{¹H}NMR spectrum of the Cr(CO)₄(PPh ₂ gives also one singlet for P centered at 74.04 ppm, this shift in ³¹P peak gives a strong evidence for the complex formation. Figure 2(a) shows the FTIR spectrum of the raw materials. The small window is the infrared spectrum of the chromium hexacarbonyl, [Cr (CO)₆]. The main features regarding the carbonyl broaden vibration group frequency is observed and centered at 1960 cm . Concerning the second spectrum in the large window which presents the triphenylphosphine (PPh raw material, the band observed at 1877 cm is very week in comparison with complex infrared spectrum which will be shown later. The remaining vibration bands are due to the aromatic rings in the PPh₃ and these bands are consistent with reported literature. Figure 2(b) shows the FTIR spectrum of the Cr(CO)₄(PPh ₂ with the mean characteristic bands at 1877, 1909, and 1918 cm . These three bands are assigned to the three different carbonyl bands in the cis-Cr(CO)₄(PPh complex, these bands consider to be a good indication of completion of complexion reaction, the band at 1089 cm is assigned to aromatic rings in the complex (P–Ar), the other two bands at 970 cm , 1280 cm , and 1433 cm were assigned to rocking bend, symmetric stretching, and asymmetric deformation of P–C groups, respectively. The Cr–P stretching frequency has been observed at about 435 cm which is definitely confirming the complexion reaction which is not seen in the raw materials. Finally, the C–H group shows symmetric and asymmetric absorptions above 3000 cm .

Fig. 2.

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	Fig. 2. The FTIR spectra of (a) the raw materials and (b) cis-tetracarbonylchromium-triphenyl and taken as KBr Disk.

The UV-Vis spectrum of Cr(CO)₄(PPh ₂ (Fig. 3) shows two maximum absorption peaks at around 235 nm and 262 nm. These transitions are assigned to and d transitions in the complex, respectively.^[19–21]

Fig. 3.

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	Fig. 3. (color online) The ultraviolet-visible spectrum of the cis-tetracarbonylchromium-triphenylphosphine in dichloromethane.

In the open aperture (OA) z-scan experiment, the Cr(CO)₄(PPh complex dissolved in dichloromethane at concentration of 10⁻³ M, with input intensity of . The transmitted intensity measured by the detector is sensitive only to the intensity variation. Here, the transmission is symmetric at the focus point ( ), where it becomes at the minimum value, the data show the dependent of the transmission with z (Fig. 4).

Fig. 4.

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	Fig. 4. (color online) The open aperture z-scan data of the tetracarbonylchromium-triphenyl phosphine complex in dichloromethane.

Theoretical fit was performed to the experimental data (Fig. 4), to estimate the nonlinear optical absorption coefficient “β” using the following equation^[6,7]

Figure 5 shows the dependence of excitation intensity ( on nonlinear absorption coefficients ( . The values of β decrease drastically with increasing I₀. This is a result of sequential two-photon absorption (TPA). In the present case, the nonlinearity is due to the TPA and excited state absorption (ESA) assisted reverse saturable absorption (RSA) process.^[22] According to the literature, the RSA process may explain on the bases of famous five-level model in the organic compounds with extended π–electron system.^[22–24] The basic condition of the RSA process is . It is well known that the β coefficient is related to by the following Eq. (4),^[25–27]

Fig. 5.

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	Fig. 5. (color online) Nonlinear coefficient β versus on-axis input intensity I0 of the tetracarbonyl chromium-triphenyl phosphine complex in dichloromethane.

To evaluate the sign and magnitude of the nonlinear refractive index n₂ of our complex, the normalized transmittance T of closed aperture (CA) is given by^[6,7,28]

Fig. 6.

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	Fig. 6. (color online) Closed aperture data of the tetracarbonyl chromium-triphenyl phosphine complex in dichloromethane.

The deduced values of the β and the n₂ were used to calculate the corresponding real and imaginary parts of the χ³ with the relations given in the literature^[6,7]

The values of α₀ were determined according to the similar method described in Ref. [18]. The linear index n₀ was obtained by using commercial Abbe refract meter. The new values of α₀, n₀, n₂, β, and are listed in the following Table 1.

Table 1.

Table 1.

Table 1. The nonlinear coefficients of tetracarbonyl chromium-triphenyl phosphine complex (Cr(CO)4(PPh3)2) in dichloromethane at concentration of 10−3 M. . Sample α0/cm n0 n2/(cm2/W) β/(cm/W) Re /esu Im /esu Cr(CO)4(PPh3)2 0. 34 1.4234

Table 1. The nonlinear coefficients of tetracarbonyl chromium-triphenyl phosphine complex (Cr(CO)4(PPh3)2) in dichloromethane at concentration of 10−3 M. .

It is clearly evident from Fig. 6 that the tetracarbonyl chromium-triphenyl phosphine exhibits peak–valley characteristics. This configuration is a direct indication of negative n₂ and ca be considered as a self-defocusing material around 635 nm. The defocusing effect rose from the absorption of the laser beam (CW) propagating through an absorbing medium. This led to variation in the n₂ where the sample works as thermal lens resulting in severe phase distortion of the propagating beam.^[27,28]

In this work, as the CW laser is used, the nonlinearity in our sample is considered as a thermal origin. The thermal nonlinearity n₂ is related to the thermo–optic by the following equation^[29]

In addition to the thermal effect, the NLO response of the tetracarbonylchromium-triphenyl phosphine complex is raised from a lot of structure factors,^[30] and the non-localized π–electron with the heavy central metal.^[31–33] The experiment was repeated to trace any contribution for the used solvent, but no signals were observed as seen in Fig. 4.

In the present paper, the nonlinear optical coefficients of our new complex — tetracarbonylchromium-triphenyl phosphine in dichloromethane compound listed in Table 1 were compared with previous results for molecules with the CW laser excitation.^[33–36]