1 Introduction

Very luminous infrared galaxies (VLIRGs) , especially ultraluminous infrared galaxies (ULIRGs) have bolometric luminosities comparable with quasars and dominate the top end of the galaxy luminosity function (Soifer et al. 1987). Many theoretical and observational studies (Sanders et al. 1988; Norris et al. 1988; Struck-Marcel & Tinsley 1978; Rieke et al. 1980; Norman & Scoville 1988; Rieke 1988; Condon et al. 1991; Leitherer & Heckman 1995) show that the high infrared luminosities are associated with the phenomena of active galactic nuclei (AGNs) and starbursts, rather than the radiation from an old stellar population (Thronson et al. 1990) and cloud-cloud collision (Harwit et al. 1987). The main energy output mechanism for luminous infrared galaxies is still being debated. Both AGNs and starbursts have been proposed as the main energy provider. Sanders et al. (1988) found that a high proportion of AGNs seems to be present in ULIRGs, but Rigopoulou et al. (1996) found that the starburst model fits well the radio, sub-millimeter/IR to soft X-ray observations for Sanders' 10 ULIRGs. Recent 3D infrared spectroscopic imaging observation (Kroker et al. 1996) suggests that both Seyfert 1 nucleus and circumnuclear star formation contribute significantly to the total luminosity. It is possible that AGNs and starbursts co-exist in the central region. Recently, Veilleux et al. (1997) reported the results of a near-infrared search for hidden broad-line regions in a sample of 25 ULIRGs with no broad-line regions in the optical band. They detected broad recombination lines in five objects and all of them are Seyfert 2 galaxies. This observation provides important clue that there could exist evolution connection from nuclear starbursts to AGNs in the central region of luminous infrared galaxies. In order to learn more about the central region of luminous infrared galaxies, and to understand the possible evolutionary process from starbursts to AGNs and how they are associated with galaxy' interaction/merging, we carried out spectroscopic observations for a large sample of 73 VLIRGs compiled from the 2 Jy redshift survey catalogue (Strauss et al. 1990, 1992). The spectra and the optical images were presented in a companion paper (Wu et al. 1998, hereafter Paper I). In this paper, we report the spectral classification and analysis for the sample (Sect. 2). The morphological classification and their environmental properties are discussed in Sect. 3. In Sect. 4, we point out a possible spectral evolutionary process of luminous infrared galaxies using the relationship between the spectral and morphological classifications. A possible nuclear and evolutionary model for these galaxies is then constructed. Throughout this paper, we use $\rm {\it H}_{0}=50~km\ s^{-1}\; Mpc^{-1}$ and $\rm {\it q}_{0}=0.5$.