The publication date is assigned to articles when they appear in the Early View of the journal. It looks like

Published on-line: 16 September 2005.

Simultaneously articles get their Digital Object Identifiers (DOI, more details see www.doi.org) looking, e.g., as "10.1002/lapl.200510054". Here is an example of a reference for the on-line published paper:

M. D'Angelo and Y.H. Shih, Laser Phys. Lett. (DOI 10.1002/lapl.200510054).

On-line published papers can be found at the publisher web site. For a given DOI, the address is http://dx.doi.org/10.1002/lapl.200510054.

On-line publication of papers precedes their publication in a printed form. After publication in a printed form, papers can be referred to in a standard manner, e.g., as

M. D'Angelo and Y.H. Shih, Laser Phys. Lett. 2, 567 (2005).

But even after publication in a printed form, on-line versions remain accessible at the publisher web site as earlier, and the above-described citation of these on-line publications with their original publication dates remain possible and legal.