I found a list of in-depth research articles from This site
. Granted, I have not actually read any of them, because the research behind Spaced Repetition (Or as Pimsleur coins it, "Graduated Interval Recall") has never been of particular interest to me.
Pimsleur's "A Memory Schedule" is accessible on the internet here
, however it requires you to purchase the right to view the full text of the article.
Asher, J.J. (1963) Evidence for "genuine" one-trial learning. IRAL 1, 2: 98-103. [11.2]
Bahrick, H.P. (1984) Semantic memory content in permastore: fifty years of memory for Spanish learned in school. Journal of Experimental Psychology: General 113, 1: 1-37. [11.2]
Bahrick, H.P. and Phelps, E. (1987) Retention of Spanish vocabulary over 8 years. Journal of Experimental Psychology: Learning, Memory and Cognition 13, 2: 344-349. [11.2]
Bahrick, H.P., Bahrick, L.E., Bahrick, A.S. and Bahrick, P.E. (1993) Maintenance of a foreign language vocabulary and the spacing effect. Psychological Science 4, 5: 316-321. [11.2]
Besson, M. and Kutas, M. (1993) The many facets of repetition a cued-recall and event-related potential analysis of repeating words in same versus different contexts. Journal of Experimental Psychology: Learning, Memory and Cognition 19, 5: 1115-1133. [11.2]
Bloom, K.C. and Shuell, T.J. (1981) Effects of massed and distributed practice on the learning and retention of second-language vocabulary. Journal of Educational Research 74, 4: 245-248. [11.2]
Dempster, F.N. (1987) Effects of variable encoding and spaced presentation on vocabulary learning. Journal of Educational Psychology 79, 2: 162-170. [11.2]
Hulme, C., Maughan, S. and Brown, G.D.A. (1991) Memory for familiar and unfamiliar words: evidence for a long-term memory contribution to short-term memory span. Journal of Memory and Language 30: 685-701. [11.2]
Kachroo, J.N. (1962) Report on an investigation into the teaching of vocabulary in the first year of English. Bulletin of the Central Institute of English 2: 67-72. [11.2]
Landauer, T.K. and Bjork, R.A. (1978) Optimum rehearsal patterns and name learning. In M.M. Gruneberg, P.E. Morris and R.N. Sykes (eds) Practical Aspects of Memory Academic Press, London: 625-632. [11.2]
Meara, P., Lightbown, P. and Halter, R.H. (1997) Classrooms as lexical environments. Language Teaching Research 1, 1: 28-47. [11.2]
Pimsleur, P. (1967) A memory schedule. Modern Language Journal 51, 2: 73-75. [11.2]